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

This function has disappeared in commit edac6f6332d9 ("Revert "net: dsa:
Allow drivers to filter packets they can decode source port from"").

Also, since commit 4e50025129ef ("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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

Documentation/networking/dsa/dsa.rst

···
 setting the ``promisc_on_master`` property of the ``struct dsa_device_ops``.
 Note that this assumes a DSA-unaware master driver, which is the norm.
 
-Hardware manufacturers are strongly discouraged to do this, but some tagging
-protocols might not provide source port information on RX for all packets, but
-e.g. only for control traffic (link-local PDUs). In this case, by implementing
-the ``filter`` method of ``struct dsa_device_ops``, the tagger might select
-which packets are to be redirected on RX towards the virtual DSA user network
-interfaces, and which are to be left in the DSA master's RX data path.
-
-It might also happen (although silicon vendors are strongly discouraged to
-produce hardware like this) that a tagging protocol splits the switch-specific
-information into a header portion and a tail portion, therefore not falling
-cleanly into any of the above 3 categories. DSA does not support this
-configuration.
-
 Master network devices
 ----------------------