staging: rtl8192u: remove r8180_pm.c and r8180_pm.h

This patch removes r8180_pm.h and r8180_pm.c from rtl8192u because they
are not used anywhere in the driver.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>