Remove unneeded negative match in .gitignore

In aa86a0dc, a line was added to .gitignore to direct Git not to ignore
the file media/redesign/css/README.md. This negative match is not needed
because Git is otherwise only ignoring stylesheets in that directory.

(When aa86a0dc was written, I mistakenly read that the entire directory
was being ignored, hence the negative match.)