Drop support for Safari 5.1

Since Safari 6.1, WebKit supports CSS3 Transitions without a prefix. Safari 5.1 has less than 0.6% market share worldwide, so dropping support for this old browser shouldn't affect compatibility for virtually anyone.

I know these tiny removals from the code probably won't make much difference in filesize, but it may make it easier to maintain the code and encourage users of the really old Safari 5.1 browser to upgrade to a later version.