vmstat: Drop vm_cnt compat hack

Prior to FreeBSD 11, vm_cnt was named cnt.  When it was renamed, vmstat
was modified to fall back to the old name if the new name was not found.
It's time we dropped this.

Reviewed by:	kib, jhb, emaste
Differential Revision:	https://reviews.freebsd.org/D59255

usr.bin/vmstat/vmstat.c

@@ -294,22 +294,11 @@ main(int argc, char *argv[])
 			xo_errx(1, "kvm_openfiles: %s", errbuf);
 	}
 
-retry_nlist:
 	if (kd != NULL && (c = kvm_nlist(kd, namelist)) != 0) {
 		if (c > 0) {
 			bufsize = 0;
 			len = 0;
 
-			/*
-			 * 'cnt' was renamed to 'vm_cnt'.  If 'vm_cnt' is not
-			 * found try looking up older 'cnt' symbol.
-			 * */
-			if (namelist[X_SUM].n_type == 0 &&
-			    strcmp(namelist[X_SUM].n_name, "vm_cnt") == 0) {
-				namelist[X_SUM].n_name = "cnt";
-				goto retry_nlist;
-			}
-
 			/*
 			 * 'nintrcnt' doesn't exist in older kernels, but
 			 * that isn't fatal.