boot: Remove stray free()

Early versions of this code had a free, but this one doesn't need
it. Remove the forgotten free(vv); from earlier versions.

Fixes:			ed56dcfc6b
Noticed by:		Michael Butler
Sponsored by:		Netflix

sys/kern/subr_boot.c

@@ -179,7 +179,6 @@ static int howto_masks[] = {
 			SETENV(n, "1");
 		else
 			SETENV(n, vv);
-		free(vv);
 	}
 #endif
 	return (howto);