LU-6142 ptlrpc: remove srv_threads from struct ptlrpc_service

LU-6142 ptlrpc: remove srv_threads from struct ptlrpc_service

The threads are not stored here - nothing is.
Threads are stored in svcpt->scp_threads.
So remove the field and update the comment.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I46875f77bb017bec47f901eca8dc946f11488cbe
Reviewed-on: https://review.whamcloud.com/35881
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: Arshad Hussain <arshad.super@gmail.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>