Linux: Remove use of undefined AFS_LINUX_ENV

Linux: Remove use of undefined AFS_LINUX_ENV

AFS_LINUX_ENV is used in a few places, mostly from commit dc077b83,
but it is not defined anywhere.  As a result the logic was not as
intended.

In the definition of osi_ThreadUnique, this means that on all
Linux we use getpid(), which is OK as it gets defined appropriately
under LINUX and LINUX24 as current->pid or current->tgid.

Change-Id: Ic8daf10480a91ff8406cd6ca3929a2ecfafa78ad
Reviewed-on: http://gerrit.openafs.org/5424
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>