fix r42527 again; [Bug #8760] [ruby-dev:47608] [ruby-dev:47609]

Remove check rl_instream != ifp->stdio_file.
rl_instream is made from duped fd and we cannnot compare.
This fix seems still buggy when rl_instream is manually changed from
other extension libraries.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42528 b2dd03c8-39d4-4d8f-98ff-823fe69b080e