Delete unused create_attachment.css

Zero results when grep'ing our modifications and upstream 4.2 code.
The file was a 1:1 copy of upstream CSS in 2009, but the upstream
CSS was moved into attachment.css as part of
https://bugzilla.mozilla.org/show_bug.cgi?id=460752 . Hence that
new file is the CSS code used when rendering in Wikimedia Bugzilla.

Bug: 54823
Change-Id: Ia2b56f6c2f760db48919ce5289b576b9e55193cc
Merge "Delete unused create_attachment.css"