cal: drop obsolete parsemonth return value checks

These became redundant since month range checks were implemented in
parsemonth in revision 1.15.

Index: cal.c
===================================================================
RCS file: /cvs/src/usr.bin/cal/cal.c,v
diff -u -p -r1.32 cal.c
--- cal.c	18 Aug 2024 19:58:35 -0000	1.32
+++ cal.c	25 Jun 2026 09:09:04 -0000
@@ -194,14 +194,10 @@ main(int argc, char *argv[])
 	switch(argc) {
 	case 2:
 		month = parsemonth(*argv++);
-		if (!month)
-			errx(1, "Unable to parse month");
 		/* FALLTHROUGH */
 	case 1:
 		if (argc == 1 && !isdigit((unsigned char)*argv[0])) {
 			month = parsemonth(*argv);
-			if (!month)
-				errx(1, "illegal year value: use 1-9999");
 			(void)time(&now);
 			local_time = localtime(&now);
 			year = local_time->tm_year + 1900;