makefs: remove vestigial '?' cases from top-level getopt(3) loop

getopt(3) returns '?' when it encounters a flag not present in the in
the optstring or if a flag is missing its option argument.  We can
handle this case with the "default" failure case with no loss of
legibility.

Obtained from:	OpenBSD makefs.c 1.22

usr.sbin/makefs/makefs.c

@@ -273,7 +273,6 @@ main(int argc, char *argv[])
 			fsoptions.sparse = 1;
 			break;
 
-		case '?':
 		default:
 			usage(fstype, &fsoptions);
 			/* NOTREACHED */