Get rid of warnings of test/spec

* lib/net/http/exceptions.rb: revert a part of r63590.  to deprecate
  Net::ProtoServerError seems to be wrong.
  see [ruby-core:87488] [Feature#14688]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63661 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/net/http/exceptions.rb

@@ -30,5 +30,4 @@ end
 
 module Net
   deprecate_constant(:HTTPServerException)
-  deprecate_constant(:ProtoServerError)
 end