webdav,srm: Do not manage life cycle of Jetty thread pool

Jetty does that itself.

Target: trunk
Request: 2.11
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7422/
(cherry picked from commit c57a0ae9d3e90ec6c9e790c8450c0bc5c551c88e)
Merge pull request #999 from gbehrmann/fix/2.11/rb7422

webdav,srm: Do not manage life cycle of Jetty thread pool