LU-13419 osc: Remove unnecessary spinlock assertion

LU-13419 osc: Remove unnecessary spinlock assertion

This spinlock assertion notably increases the time spent in
getting grant with many threads to few OSCs, but adds
little value.  Remove it.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <paf0187@gmail.com>
Change-Id: I596b9ac349a6dd1a633c6671d19d15bbf61b9fa2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56957
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Zhenyu Xu <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>