Remove task exclusive mode from ns_clientmgr [v9.18]

The .lock, .exiting and .excl members were not using for anything else than starting task exclusive mode, setting .exiting to true and ending exclusive mode.

Remove all the stray members and dead code eliminating the task exclusive mode use from ns_clientmgr.

(cherry picked from commit 4f74e101)

Closes #3230 (closed)