network/brvlan: remove unnecessary bridge vlan IDs

network/brvlan: remove unnecessary bridge vlan IDs

When an interface is being reconfigured with different bridge vlan
settings, this makes old vlan IDs on the interface removed.

This also makes the PVID= setting support negative boolean value, e.g. "no",
in which case, the currently assigned PVID (typically, assigned by the
kernel when an interface is joined to a bridge) is dropped.
This feature is requested by #15291.

Note, if a .network file has no settings about bridge vlan, networkd
keeps the currently assigned vlan IDs. That's intended, to make not
break existing setups.
When a .network file has only PVID=no line in [BridgeVLAN] section, then
all assigned vlan IDs are removed.

Fixes #29975.
Closes #15291.