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.

Source/cmCacheManager.cxx

@@ -750,12 +750,8 @@ void cmCacheManager::AddCacheEntry(const char* key,
     }
   e.SetProperty("HELPSTRING", helpString? helpString :
                 "(This variable does not exist and should not be used)");
-  if (this->Cache[key].Value == e.Value)
-    {
   this->CMakeInstance->UnwatchUnusedCli(key);
 }
-  this->Cache[key] = e;
-}
 
 bool cmCacheManager::CacheIterator::IsAtEnd() const
 {