msm: ssr: Remove entry from subsys list on unregistering

Currently, on a subsystem being unregistered, its entry is not
removed from the subsys list. This causes a crash when this
list is accessed at a later time.
Remove the subsys entry from the list during unregister.

Change-Id: I61f3cb82bf1ac22c6ef1374e530e14ccaaa095bb
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>