webdav: Exclude unused milton dependencies

The milton library also supports CalDAV. We do not utilize this
functionality in dCache and can thus exclude dependencies introduced
by it.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/6114/