Remove checks for username and password.

[vanniktech]

Fixes #11

[codecov]

Codecov Report

Merging #12 into master will increase coverage by 4.13%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master      #12      +/-   ##
============================================
+ Coverage     88.88%   93.02%   +4.13%     
  Complexity        6        6              
============================================
  Files             2        2              
  Lines            90       86       -4     
  Branches          8        6       -2     
============================================
  Hits             80       80              
+ Misses            3        1       -2     
+ Partials          7        5       -2

Impacted Files	Coverage Δ	Complexity Δ
...vanniktech/maven/publish/MavenPublishPlugin.groovy	97.53% <ø> (+4.58%)	1 <0> (ø)	⬇️

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