USB: Remove unused LPM variable.

hub_initiated_lpm_disable_count is not used by any code, so remove it.

This commit should be backported to kernels as old as 3.5, that contain
the commit 8306095f

 "USB: Disable USB
3.0 LPM in critical sections."

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org