spacemanager: Use PoolMonitor to extract link groups

Since PoolMonitor is imported every 30 seconds anyway, we don't
need to ask pool manager for link groups anymore.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6377/