bridge: mcast: Remove redundant checks

These checks are now redundant as they are performed by
br_mdb_config_init() while parsing the RTM_{NEW,DEL}MDB messages.

Remove them.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>