USB / PM: Remove unneeded #ifdef and associated dead code

In commit ceb6c9c8

 ("USB / PM: Drop CONFIG_PM_RUNTIME from the
USB core"), all occurrences of CONFIG_PM_RUNTIME in the USB core
code were replaced by CONFIG_PM. This created the following structure
of #ifdef blocks in drivers/usb/core/hub.c:

 [...]
 #ifdef CONFIG_PM
 #ifdef CONFIG_PM
 /* always on / undead */
 #else
 /* dead */
 #endif
 [...]

This patch removes unnecessary inner "#ifdef CONFIG_PM" as well as
the corresponding dead #else block. This inconsistency was found using
the undertaker-checkpatch tool.

Signed-off-by: Andreas Ruprecht <rupran@einserver.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>