Ditch all @since DocBlocks from theme.

@since as a DocBlock in _s doesn't make much sense and should be removed entirely from the theme. Reasons:

1. The DocBlock makes it hard to retrofit old themes to _s without going through the trouble of manually ditching each @since DocBlock
2. In practice new functions rarely, if ever, get added into the themes we're developing using _s and @since feels like unnecessary clutter.
3. @since _s 1.0 is unique to _s itself, and doesn't make sense in context with other themes. For example, '@since _s 1.2' in the Customizer makes sense for _s but when a new theme is spun off using _s, that version number loses meaning and needs a new version number for the theme that's being developed.
4. I'd prefer to look at _s as a non-versioned theme that keeps getting rolling updates to its V. 1.0.

I'm certain there are other good reasons to gut this, but my brain is failing me at the moment. I think the above three are more than adequate.