Revert "workqueue: make wq_subsys const"

This reverts commit d412ace1. This leads to
build failures as it depends on a driver-core commit 32f78abe

 ("driver
core: bus: constantify subsys_register() calls"). Let's drop it from wq tree
and route it through driver-core tree.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202402051505.kM9Rr3CJ-lkp@intel.com/