ion/system_heap: Revisit secure pool shrinker implementation

The previous implementation of moving pages to the uncached pool before
freeing them is unnecessarily complex.
Follow the same methodology as ion_page_pool_shrink().

Change-Id: Ic69947c18b5899e5ca75b7e0039e4e2869a5dfa6
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>