LU-2622 obdclass: Remove cl_env cache

This patch removes the cl_env cache, forcing each thread needing one of
these structure to create a new one.

Test-Parameters: fortestonly
Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: Ief4b524784e07d7677ecb8a9ce97a7b54ccc6f75