pom: Fix duplicate include of release plugin

Commit 5a5ccd2d5c3353331881dc5df0645a429df836e9 introduced lots
of errors in the Maven build caused by duplicate declaration of
the release plugin.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6491/