cmCacheManager: Avoid cache entry self-assignment
Since commit bef93dc5 (Couple of changes: cache variables now have a map of properties, 2002-09-11) the cmCacheManager::AddCacheDefinition method accesses its map entry by reference. However, the commit left the original entry assignment at the end of the method. With Apple Clang 5.1 and libc++ this self-assignment destroys the cache entry property map. Drop the self assignment. Also drop the condition around the call to UnwatchUnusedCli since it was a self-comparison that must always have been true.