Remove dead RawText(RawTextComparator) constructor

Since the introduction of HashedSequence we no longer need to supply
the RawTextComparator at the time of constructing a RawText.  Drop the
definition from the constructor, because it doesn't make sense as part
of our public API.

Change-Id: Iaab34611d60eee4a2036830142b089b2dae81842
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>