Remove reference to multipath algorithms in usage

IP multipath algorithms support was removed several revisions ago.
Remove from usage as well

ip/iproute.c

@@ -82,7 +82,6 @@ static void usage(void)
 	fprintf(stderr, "          unreachable | prohibit | blackhole | nat ]\n");
 	fprintf(stderr, "TABLE_ID := [ local | main | default | all | NUMBER ]\n");
 	fprintf(stderr, "SCOPE := [ host | link | global | NUMBER ]\n");
-	fprintf(stderr, "MP_ALGO := { rr | drr | random | wrandom }\n");
 	fprintf(stderr, "NHFLAGS := [ onlink | pervasive ]\n");
 	fprintf(stderr, "RTPROTO := [ kernel | boot | static | NUMBER ]\n");
 	fprintf(stderr, "TIME := NUMBER[s|ms]\n");