srm: Simplify unpin callbacks

The callback was essentially only used for logging, and in three
of the four cases the unpin call was blocking already and didn't
need a callback.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6955/