[MemInit] Remove process_disable_memory_mitigations

This function is no longer necessary, and uses a deprecated API
(mallopt(M_BIONIC_DISABLE_MEMORY_MITIGATIONS)). This function was
provided originally as part of libcutils because the original API was a
private android_mallopt.

The API is being reborn as a memory-init-specific mallopt. Given that
regular system components and apps will be able to call it directly,
it's not necessary to have a wrapper function in libcutils any more.

Bug: 135772972
Test: Build the device.
Change-Id: I4a3185b99702c8e445de6837b154c4a96c8295a1