Remove strscan dependency declaration from gemspec (#204)

`strscan` is a part of the Ruby standard library in all versions of Ruby supported by REXML.
So we don't need to declare it as a dependency explicitly.

See also: #140 (comment)

rexml.gemspec

@@ -58,6 +58,4 @@ Gem::Specification.new do |spec|
   spec.extra_rdoc_files = rdoc_files
 
   spec.required_ruby_version = '>= 2.5.0'
-
-  spec.add_runtime_dependency("strscan")
 end