net: bridge: multicast: handle block pg delete for all cases

A block report can result in empty source and host sets for both include
and exclude groups so if there are no hosts left we can safely remove
the group. Pull the block group handling so it can cover both cases and
add a check if EHT requires the delete.

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>