net: bridge: multicast: add EHT host delete function

Now that we can delete set entries, we can use that to remove EHT hosts.
Since the group's host set entries exist only when there are related
source set entries we just have to flush all source set entries
joined by the host set entry and it will be automatically removed.

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