afs: remove commented xstats externs

afs: remove commented xstats externs

Extern declarations for the xstats recording areas have been commented
out since 8f2df21ffe59e9aa66219bf24656775b584c122d
"pull-prototypes-to-head-20020821".

Remove the vestigial comments.

No functional change is incurred by this commit.

Change-Id: Ieef9a4b21e78db8d5427bed7b621ba043663b1d1
Reviewed-on: https://gerrit.openafs.org/14197
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>