Remove StdErrSplitter workarounds

Kept gems warning muted due to:

1.

      RuntimeError:
       Warnings were generated: /home/runner/work/rspec-support/bundle/ruby/2.5.0/gems/actioncable-6.0.3.4/lib/action_cable/channel/test_case.rb:178: warning: method redefined; discarding old connection
     Shared Example Group: "an rspec-rails example group mixin" called from ./spec/rspec/rails/example/channel_example_group_spec.rb:6
     # /home/runner/work/rspec-support/rspec-support/lib/rspec/support/spec/stderr_splitter.rb:38:in `verify_no_warnings!'

2.
       RuntimeError:
        Warnings were generated: /home/runner/work/rspec-support/bundle/ruby/2.5.0/gems/actionpack-6.0.3.4/lib/action_dispatch/routing/route_set.rb:560: warning: instance variable @_routes not initialized
      Shared Example Group: "an rspec-rails example group mixin" called from ./spec/rspec/rails/example/view_example_group_spec.rb:5
      # /home/runner/work/rspec-support/rspec-support/lib/rspec/support/spec/stderr_splitter.rb:38:in `verify_no_warnings!'

Author: pirj

lib/rspec/support/spec/stderr_splitter.rb

@@ -27,15 +27,8 @@ def reopen(*args)
         @orig_stderr.reopen(*args)
       end
 
-      # To work around JRuby error:
-      # can't convert RSpec::Support::StdErrSplitter into String
-      def to_io
-        @orig_stderr.to_io
-      end
-
-      # To work around JRuby error:
-      # TypeError: $stderr must have write method, RSpec::StdErrSplitter given
       def write(line)
+        # Ignore warnings coming from gems, specifically Rails on Ruby 2.5+
         return if line =~ %r{^\S+/gems/\S+:\d+: warning:} # http://rubular.com/r/kqeUIZOfPG
 
         # Ruby 2.7.0 warnings from keyword argments span multiple lines, extend check above