LU-12034 obdclass: disable server-only code on client

LU-12034 obdclass: disable server-only code on client

The lu_env_add(), lu_env_remove(), and lu_env_find() functions are
only used on the server.  Conditionally remove them when doing a
client-only build.

Fixes: fce8d80624fd ("LU-12034 obdclass: put all service env on list")

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I15f4d4de583bb3f9d16adad3ea16f961853ebbe5
Reviewed-on: https://review.whamcloud.com/35566
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>