power_domain: Remove thread

This patch removes the creation of a thread for this
module.

The separate thread was temporarily added to support drivers not
capable to handle asynchronously requests. Now that these drivers
have been updated, this can be removed.

Change-Id: I72cd4f0cda2ec25ab3d552ae63f5377ec5bf802b
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>