LU-12965 obdclass: remove assertion for imp_refcount

LU-12965 obdclass: remove assertion for imp_refcount

After calling obd_zombie_import_add(), obd_import could
be freed by obd_zombie before we check imp_refcount with
LASSERT_ATOMIC_GE_LT. It's a use after free and could
crash the box.

Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Change-Id: I3d63acf2bff543924ca0e74a35d24c507d68f6aa
Reviewed-on: https://review.whamcloud.com/36743
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>