poolmanager: Remove link group attributes

Link group attributes were not used. The following commands have
been removed from the command line interface:

psu remove linkGroup attribute
psu set linkGroup attribute

The latter of these still exists in the code as a noop to alove existing
poolmanager.conf files to be read.

Target: trunk
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6418/