net: dsa: do not check orig_dev in vlan del

The current DSA code handling switchdev objects does not recurse into
the lower devices thus is never called with an orig_dev member being
a bridge device, hence remove this useless check.

At the same time, remove the comments about the callers, which is
unlikely to be updated if the code changes and thus will be confusing.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>