Remove SkOmxPixelRef to fix build.

Recent changes to SkPixelRef (from which SkOmxPixelRef inherited)
added abstract functions which SkOmxPixelRef does not implement.

Solution: remove SkOmxPixelRef, which is completely unused. The only
code that created one was commented out at the same time as it was
added in 2009, so there's likely no benefit to reviving it.

Change-Id: Icbf2537fcc03fd464fd791ff91659a1812019623