Documentation: networking: dsa: Remove priv_size description

We no longer have a priv_size structure member since 5feebd0a ("net:
dsa: Remove allocation of driver private memory")
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/dsa/dsa.txt

@@ -369,8 +369,6 @@ does not allocate any driver private context space.
 Switch configuration
 --------------------
 
-- priv_size: additional size needed by the switch driver for its private context
-
 - tag_protocol: this is to indicate what kind of tagging protocol is supported,
   should be a valid value from the dsa_tag_protocol enum