workqueue: remove CPU offline trustee

With the previous changes, a disassociated global_cwq now can run as
an unbound one on its own - it can create workers as necessary to
drain remaining works after the CPU has been brought down and manage
the number of workers using the usual idle timer mechanism making
trustee completely redundant except for the actual unbinding
operation.

This patch removes the trustee and let a disassociated global_cwq
manage itself.  Unbinding is moved to a work item (for CPU affinity)
which is scheduled and flushed from CPU_DONW_PREPARE.

This patch moves nr_running clearing outside gcwq and manager locks to
simplify the code.  As nr_running is unused at the point, this is
safe.

Change-Id: I8288bc1cd8bd73d97bcead52d0b3b2e5921b2fec
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>