Remove Warning.warn override in MSpec and --warnings option

* Overriding Warning.warn cause several problems, including it's much
  harder and confusing to test Warning.warn in specs. Notably caused by
  Warning.warn in 2.7 not supporting the category: keyword argument.
* specs fail when $VERBOSE is true globally, so there is no real value
  to filter those $VERBOSE=true-only warnings.
* mspec requires ruby 2.7+, which has Warning.warn, so
  `$VERBOSE = nil unless ENV['OUTPUT_WARNINGS']` was dead code,
  and so the --warnings option had no effect.

lib/mspec/commands/mspec.rb

@@ -38,11 +38,6 @@ def options(argv = ARGV)
 
     options.targets
 
-    options.on("--warnings", "Don't suppress warnings") do
-      config[:flags] << '-w'
-      ENV['OUTPUT_WARNINGS'] = '1'
-    end
-
     options.on("-j", "--multi", "Run multiple (possibly parallel) subprocesses") do
       config[:multi] = true
     end

lib/mspec/helpers/io.rb

@@ -7,7 +7,7 @@ def initialize
   end
 
   def write(*str)
-    self << str.join
+    self << str.join('')
   end
 
   def << str
@@ -16,7 +16,7 @@ def << str
   end
 
   def print(*str)
-    write(str.join + $\.to_s)
+    write(str.join('') + $\.to_s)
   end
 
   def method_missing(name, *args, &block)

lib/mspec/matchers/complain.rb

@@ -19,15 +19,13 @@ def matches?(proc)
     @verbose = $VERBOSE
     err = IOStub.new
 
-    Thread.current[:in_mspec_complain_matcher] = true
     $stderr = err
     $VERBOSE = @options.key?(:verbose) ? @options[:verbose] : false
     begin
       proc.call
     ensure
       $VERBOSE = @verbose
       $stderr = @saved_err
-      Thread.current[:in_mspec_complain_matcher] = false
     end
 
     @warning = err.to_s

lib/mspec/utils/warnings.rb

@@ -8,46 +8,3 @@
   Warning[:deprecated] = true
   Warning[:experimental] = false
 end
-
-if Object.const_defined?(:Warning) and Warning.respond_to?(:warn)
-  def Warning.warn(message, category: nil)
-    # Suppress any warning inside the method to prevent recursion
-    verbose = $VERBOSE
-    $VERBOSE = nil
-
-    if Thread.current[:in_mspec_complain_matcher]
-      return $stderr.write(message)
-    end
-
-    case message
-    # $VERBOSE = true warnings
-    when /(.+\.rb):(\d+):.+possibly useless use of (<|<=|==|>=|>) in void context/
-      # Make sure there is a .should otherwise it is missing
-      line_nb = Integer($2)
-      unless File.exist?($1) and /\.should(_not)? (<|<=|==|>=|>)/ === File.readlines($1)[line_nb-1]
-        $stderr.write message
-      end
-    when /possibly useless use of (\+|-) in void context/
-    when /assigned but unused variable/
-    when /method redefined/
-    when /previous definition of/
-    when /instance variable @.+ not initialized/
-    when /statement not reached/
-    when /shadowing outer local variable/
-    when /setting Encoding.default_(in|ex)ternal/
-    when /unknown (un)?pack directive/
-    when /(un)?trust(ed\?)? is deprecated/
-    when /\.exists\? is a deprecated name/
-    when /Float .+ out of range/
-    when /passing a block to String#(bytes|chars|codepoints|lines) is deprecated/
-    when /core\/string\/modulo_spec\.rb:\d+: warning: too many arguments for format string/
-    when /regexp\/shared\/new_ascii(_8bit)?\.rb:\d+: warning: Unknown escape .+ is ignored/
-    else
-      $stderr.write message
-    end
-  ensure
-    $VERBOSE = verbose
-  end
-else
-  $VERBOSE = nil unless ENV['OUTPUT_WARNINGS']
-end

spec/commands/mspec_spec.rb

@@ -92,33 +92,6 @@
   end
 end
 
-RSpec.describe "The --warnings option" do
-  before :each do
-    @options, @config = new_option
-    allow(MSpecOptions).to receive(:new).and_return(@options)
-    @script = MSpecMain.new
-    allow(@script).to receive(:config).and_return(@config)
-  end
-
-  it "is enabled by #options" do
-    allow(@options).to receive(:on)
-    expect(@options).to receive(:on).with("--warnings", an_instance_of(String))
-    @script.options
-  end
-
-  it "sets flags to -w" do
-    @config[:flags] = []
-    @script.options ["--warnings"]
-    expect(@config[:flags]).to include("-w")
-  end
-
-  it "set OUTPUT_WARNINGS = '1' in the environment" do
-    ENV['OUTPUT_WARNINGS'] = '0'
-    @script.options ["--warnings"]
-    expect(ENV['OUTPUT_WARNINGS']).to eq('1')
-  end
-end
-
 RSpec.describe "The -j, --multi option" do
   before :each do
     @options, @config = new_option