res_sorcery_memory_cache: Change an error to a debug message

When a sorcery user calls ast_sorcery_delete on an object that
may have already expired from the cache, res_sorcery_memory_cache
spits out an ERROR.  Since this can happen frequently and validly when
an inbound registration expires after the cache entry expired, the
errors are unnecessary and misleading.  Changed to a debug/1.

Change-Id: Idf3a67038c16e3da814cf612ff4d6d18ad29ecd7

res/res_sorcery_memory_cache.c

@@ -1557,7 +1557,7 @@ static int sorcery_memory_cache_delete(const struct ast_sorcery *sorcery, void *
 	ao2_unlock(cache->objects);
 
 	if (res) {
-		ast_log(LOG_ERROR, "Unable to delete object '%s' from sorcery cache\n", ast_sorcery_object_get_id(object));
+		ast_debug(1, "Unable to delete object '%s' from sorcery cache\n", ast_sorcery_object_get_id(object));
 	}
 
 	return res;