Drop deprecated fields

Over time we have accumulated a number of fields we have kept
for backwards compatibility with old pools. Current master
only needs to maintain compatibility with 2.10 and 2.11, so
there is no need to keep these old fields anymore.

The obsolete 'psu set linkGroup attribute' command is removed
too. Sites are adviced to 'save' their pool manager config
prior to upgrading to ensure that this command is no longer
present in the configuration.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: https://rb.dcache.org/r/7604/