vol: use OS_UNLINK instead of unlink

vol: use OS_UNLINK instead of unlink

Use OS_UNLINK in order to ensure that platform specific
unlink functions are called.  OS_UNLINK is unlink on Unix.

Reviewed-on: http://gerrit.openafs.org/3704
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 089a3f5e8b0b00942cd0ccff5be7b4f671fcbf82)

Change-Id: Ib3207b7c365b9ede5299bdbc7626e63450198ad7
Reviewed-on: http://gerrit.openafs.org/3851
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>