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.

Reviewed-on: http://gerrit.openafs.org/10581
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 0c5d403076946c7a4f53f5e44df8fe0d986cea06)

Change-Id: Ie686545c6618242f2c2644a01891cd0dfe0d0d05
Reviewed-on: http://gerrit.openafs.org/10600
Reviewed-by: Ken Dreyer <ktdreyer@ktdreyer.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Tested-by: BuildBot <buildbot@rampaginggeek.com>