LU-6142 ptlrpc: remove scp_nthrs_stopping field.

LU-6142 ptlrpc: remove scp_nthrs_stopping field.

This field is unused, so remove it.
If "shrinking threads" is ever needed, any extra fields
required can be added then.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: Ie727033133aa846d92fdc52f6545bc1cd5efe153
Reviewed-on: https://review.whamcloud.com/35882
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>