ion: Remove secure and unsecure buffer APIs

These APIs are deprecated as securing and unsecuring for secure
buffers is done on allocation and free respectively by the secure
heap layer. Clients don't have to call these explicitly.

Change-Id: If88cd1c47cba346446ebdcad494850b54ba954ab
Signed-off-by: Shalaj Jain <shalajj@codeaurora.org>