LU-18141 obd: remove OBD_SLAB_FREE_RTN0()

LU-18141 obd: remove OBD_SLAB_FREE_RTN0()

This macro isn't called anywhere. So let's
remove it.

Test-Parameters: trivial
Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: I485d1d40220a2cd1a65c430e6487ad2d2052ec44
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56051
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: jsimmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>