OpenBSD: Remove duplicate assignment of COMMON_INCLUDE in libafs

OpenBSD: Remove duplicate assignment of COMMON_INCLUDE in libafs

When building libafs, the make file variable COMMON_INCLUDE is
assigned a value in two places: Makefile.common.in and
MakefileProto.OBSD.in. The assignment in the latter place is both
wrong and unnecessary. Remove it.

Reviewed-on: http://gerrit.openafs.org/3430
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit ba63d22bfb0b18ab054c39cafafeb8c6881fe687)

Change-Id: I08d46d7e9c288e6b81f8c9cf3a16f6e48ac3462f
Reviewed-on: http://gerrit.openafs.org/3722
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>