tcltklib.c: revert RUBY_UNTYPED_DATA_WARNING

* ext/tk/tcltklib.c (RUBY_UNTYPED_DATA_WARNING): Data_Get_Struct
  is not affected.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48694 b2dd03c8-39d4-4d8f-98ff-823fe69b080e