ashmem: remove cache maintenance support

The cache maintenance routines in ashmem were causing
several security issues. Since they are not being used
anymore by any drivers, its well to remove them entirely.

CRs-Fixed: 1107034, 2001129, 2007786
Change-Id: I955e33d90b888d58db5cf6bb490905283374425b
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>