timers/migration: Prevent from lockdep false positive warning

Testing housekeeping_cpu() will soon require that either the RCU "lock"
is held or the cpuset mutex.

When CPUs get isolated through cpuset, the change is propagated to
timer migration such that isolation is also performed from the migration
tree. However that propagation is done using workqueue which tests if
the target is actually isolated before proceeding.

Lockdep doesn't know that the workqueue caller holds cpuset mutex and
that it waits for the work, making the housekeeping cpumask read safe.

Shut down the future warning by removing this test. It is unecessary
beyond hotplug, the workqueue is already targeted towards isolated CPUs.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Gabriele Monaco <gmonaco@redhat.com>