refperf: Remove redundant kfree() after torture_stop_kthread()

Remove unnecessary kfree(main_task) call in ref_scale_cleanup() as
torture_stop_kthread() already handles the memory cleanup for the
task structure internally.

The additional kfree(main_task) call after torture_stop_kthread()
is redundant and confusing since torture_stop_kthread() sets the
pointer to NULL, making this a no-op.

This pattern is consistent with other torture test modules where
torture_stop_kthread() is called without explicit kfree() of the
task pointer, as the torture framework manages the task lifecycle
internally.

Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>