Remove useless check; vm_pindex_t is unsigned on all architectures.

CID:		3701
Found with:	Coverity Prevent

sys/vm/vm_pageout.c

@@ -1895,7 +1895,7 @@ vm_daemon()
 				continue;
 
 			size = vmspace_resident_count(vm);
-			if (limit >= 0 && size >= limit) {
+			if (size >= limit) {
 				vm_pageout_map_deactivate_pages(
 				    &vm->vm_map, limit);
 			}