docs: net: dsa: remove references to struct dsa_device_ops::filter

This function has disappeared in commit edac6f63 ("Revert "net: dsa:
Allow drivers to filter packets they can decode source port from"").

Also, since commit 4e500251

 ("net: dsa: generalize overhead for
taggers that use both headers and trailers"), the next paragraph is no
longer true (it is still discouraged to do that, but it is now
supported, so no point in mentioning it). Delete.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>