LU-6142 osc: minor function cleanups.

LU-6142 osc: minor function cleanups.

osc_cleanup() doesn't exist, so remove declaration.

Return value of osc_extent_release() is always zero and never used, so
change it to 'void'.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I048e4ec1ec62d2b576f2a1e3c6148c0121cd91b6
Reviewed-on: https://review.whamcloud.com/39737
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>