ion: disable system contig heap

A malicious application can take advantage of the ION contig heap to
create a specific memory chunk size to exercise a rowhammer attack on the
physical hardware.
So remove support for the ION contig heap.

Change-Id: I9cb454cebb74df291479cecc3533d2c684363f77
Signed-off-by: Liam Mark <lmark@codeaurora.org>