Minileven: remove WP 3.4 backward compatibility

See #301
Likes: remove pre-3.8 styles

See #301
Likes: remove pre-mp6 filter

See #301
General: no need to add an mp6 class anymore
Likes: remove another mp6 filter
Omnisearch: remove pre-mp6 CSS
Contact Form: remove pre-mp6 CSS
Merge pull request #981 from jeherve/update/old-version-compare

General: remove No longer necessary backwards compat code