net: dst: remove gc leftovers

Get rid of some obsolete gc-related documentation and macros that were
missed in commit 5b7c9a8ff828 ("net: remove dst gc related code").

CC: Wei Wang <weiwan@google.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Acked-by: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index a5da63e..14c7fda 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1176,7 +1176,7 @@ static struct dst_entry *ipv4_dst_check(struct dst_entry *dst, u32 cookie)
 	 *
 	 * When a PMTU/redirect information update invalidates a route,
 	 * this is indicated by setting obsolete to DST_OBSOLETE_KILL or
-	 * DST_OBSOLETE_DEAD by dst_free().
+	 * DST_OBSOLETE_DEAD.
 	 */
 	if (dst->obsolete != DST_OBSOLETE_FORCE_CHK || rt_is_expired(rt))
 		return NULL;