hwui: Fix a sporadic crash seen during CTS runs

 * It's possible (due to race conditions) that a patch might be
   scheduled for removal twice. Just print a warning and continue
   instead of asserting and crashing.

Change-Id: I42865f6717816a743cc49ba15b839e6c3b57ed83
(cherry picked from commit ddc230eaeef586cb8cf6fc128fd68268a6d6ffe4)