utils: use g_str_has_prefix() instead of prefixcmp()

Remove duplicated code from MPD.

src/utils.c

@@ -261,13 +261,3 @@ void xpthread_cond_destroy(pthread_cond_t *cond)
 	if ((err = pthread_cond_destroy(cond)))
 		FATAL("failed to destroy cond: %s\n", strerror(err));
 }
-
-int prefixcmp(const char *str, const char *prefix)
-{
-	for (; ; str++, prefix++)
-		if (!*prefix)
-			return 0;
-		else if (*str != *prefix)
-			return (unsigned char)*prefix - (unsigned char)*str;
-}
-