net: mscc: ocelot: don't reset the pvid to 0 when deleting it

I have no idea why this code is here, but I have 2 hypotheses:

1.
A desperate attempt to keep untagged traffic working when the bridge
deletes the pvid on a port.

There was a fairly okay discussion here:
https://lore.kernel.org/netdev/CA+h21hrRMrLH-RjBGhEJSTZd6_QPRSd3RkVRQF-wNKkrgKcRSA@mail.gmail.com/#t


which established that in vlan_filtering=1 mode, the absence of a pvid
should denote that the ingress port should drop untagged and priority
tagged traffic. While in vlan_filtering=0 mode, nothing should change.

So in vlan_filtering=1 mode, we should simply let things happen, and not
attempt to save the day. And in vlan_filtering=0 mode, the pvid is 0
anyway, no need to do anything.

2.
The driver encodes the native VLAN (ocelot_port->vid) value of 0 as
special, meaning "not valid". There are checks based on that. But there
are no such checks for the ocelot_port->pvid value of 0. In fact, that's
a perfectly valid value, which is used in standalone mode. Maybe there
was some confusion and the author thought that 0 means "invalid" here as
well.

In conclusion, delete the code*.

*in fact we'll add it back later, in a slightly different form, but for
an entirely different reason than the one for which this exists now.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>