net: bridge: mcast: factor out port group del

In order to avoid future errors and reduce code duplication we should
factor out the port group del sequence. This allows us to have one
function which takes care of all details when removing a port group.

v4: set pg's fast leave flag when deleting due to fast leave
    move the patch before adding source lists

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