net: dst: remove gc leftovers

Get rid of some obsolete gc-related documentation and macros that were
missed in commit 5b7c9a8f

 ("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>