Garbage-collect afs_GCUserData's argument

We no longer need the ability to force all rxnull connections to be
reaped, as the epoch is set globally.  Change the prototype and
callers accordingly.

Change-Id: I0815fdd035c3dd9d56f72e43603f9c53f5cec79d
Reviewed-on: http://gerrit.openafs.org/10842
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>