Delete PatienceDiff

HistogramDiff outperforms it for any case where PatienceDiff needs to
fallback to another algorithm.  Consequently it's not worth keeping
around, because we would always want a fallback enabled.

Change-Id: I39b99cb1db4b3be74a764dd3d68cd4c9ecd91481
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>