Clean up dead clear() code in SkRecord.

This removes the SkRecords::Clear struct and everything that refers to it.
Notice there is nothing actually creating a Clear, which means this is all
dead code.

Now that all ops obey the clip, I don't think we need the weird
inflate-empty-to-epsilon hack for BBH queries.

BUG=skia:

Review URL: https://codereview.chromium.org/835813002