Twenty Thirteen: remove custom comment callback in favor of core `comment_form()` defaults. Props obenland, closes #22005. See #20088.

git-svn-id: http://core.svn.wordpress.org/trunk@23696 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: a bit wider search input in main content area. Props obenland, fixes #23765.

Also clean out one case of unneeded "px" value unit.

git-svn-id: http://core.svn.wordpress.org/trunk@23697 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Thirteen: remove several more `px` unit values.

git-svn-id: http://core.svn.wordpress.org/trunk@23698 1a063a9b-81f0-0310-95a4-ce76da25c4cd