Twenty Thirteen: remove CSS hack added in r24286 for pixel shift in Chrome with gallery thumbs. Not needed now that we're loading medium size (and not large), see r24338.

git-svn-id: https://core.svn.wordpress.org/trunk@24350 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: PHPDoc cleanup/completion, props DrewAPicture. Fixes #24413.

git-svn-id: https://core.svn.wordpress.org/trunk@24351 1a063a9b-81f0-0310-95a4-ce76da25c4cd