win32/file.c: refine convert_mb_to_wchar

* win32/file.c (convert_mb_to_wchar): use bare pointer instead of
  VALUE, and remove useless argument.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42179 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
win32/file.c: refine fix_string_encoding

* win32/file.c (fix_string_encoding): simplify with rb_str_conv_enc().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42180 b2dd03c8-39d4-4d8f-98ff-823fe69b080e