make openafs uninstallable even if /afs is missing

The preuninstall scriptlet of the openafs RPM removes /afs. If, for
whatever reason, that directory does not exist, the scriptlet will
fail and hence break the deinstallation of the openafs package. The
proposed patch makes the scriptlet evaluate to true even if the /afs
has been removed by some other means and allows the package to be
uninstalled.

Change-Id: I3340c94521e15c56fe10840aff7b0b1080009c10
Reviewed-on: http://gerrit.openafs.org/10581
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>