afs: remove vestigial externs for afs_xvcache

afs: remove vestigial externs for afs_xvcache

These have not been needed since src/afs/afs_prototypes.h gained 'extern
afs_rwlock_t afs_xvcache' with commit
8f2df21ffe59e9aa66219bf24656775b584c122d
"pull-prototypes-to-head-20020821"

Remove the vestigial extern references.

Change-Id: Id6aceff0d5df1f1bed210a3fbf2951c62f35ddbb
Reviewed-on: https://gerrit.openafs.org/14406
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>