LU-10994 clio: remove cpl_obj

LU-10994 clio: remove cpl_obj

Remove cpl_obj from struct cl_page_slice. This member is only used in
the osc layer and struct osc_page already contains a pointer to the
osc_object.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I6451aa50ff0e8db67f1c6f4f7edbde4fa8d36c5b
Reviewed-on: https://review.whamcloud.com/47402
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>