dselect-ftp: stop calling dpkg --forget-old-unavail as it's a no-op

And it outputs an annoying warning nowadays.

dselect/methods/ftp/update

@@ -242,10 +242,5 @@ if (!$packages_modified) {
 	    $exit = 1;
 	}
     }
-
-    if(system("dpkg", "--forget-old-unavail")) {
-	print "dpkg --forget-old-unavail failed";
-	die "error";
-    }
 }
 exit $exit;