cachedb_mongodb: Avoid redundant mongoc_init() calls

mongoc_init() is using pthread_once() anyway, so we might as well
avoid calling it multiple times.

(cherry picked from commit 7a5e2c7)

modules/cachedb_mongodb/cachedb_mongodb.c

@@ -142,8 +142,6 @@ static int child_init(int rank)
 		return 0;
 	}
 
-	mongoc_init();
-
 	for (it = mongodb_script_urls;it;it=it->next) {
 		LM_DBG("iterating through conns - [%.*s]\n",it->url.len,it->url.s);
 		con = mongo_con_init(&it->url);