ruby/ruby.h: eliminate disabled function call

* include/ruby/ruby.h (RUBY_SAFE_LEVEL_CHECK): eliminate function
  call for warning/error if not match to get rid of unconditional
  warning/error by a certain compiler option.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47866 b2dd03c8-39d4-4d8f-98ff-823fe69b080e