Un-export opr_AssertFailU

Un-export opr_AssertFailU

It appears to have been created for parity with osi_AssertFailU, but
was then never used.

It is safe to remove the export line, since this export has never
been in a released version of OpenAFS.

Reviewed-on: https://gerrit.openafs.org/13212
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 691757576fb6d60a34fef2c4bc50ae581b65ad76)

Change-Id: I4811d1587fc7a25e90e2a0203d965ef352efe7c5
Reviewed-on: https://gerrit.openafs.org/13214
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>