vol: use OS_UNLINK() instead of unlink()

vol: use OS_UNLINK() instead of unlink()

OS_UNLINK provides the platform specific mechanism
for unlinking objects.

Reviewed-on: http://gerrit.openafs.org/3670
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 038264b4d62803bd71ac0d7706b56b7f18f31dca)

Change-Id: I58a6acbe7328faa1d0118073f24321ead378efe4
Reviewed-on: http://gerrit.openafs.org/3834
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>