get rid of MemoryHeapPmem

MemoryHeapPmem is not used any longer. PMEM is not a supported
type of memory by the system anymore. a particular device might
use PMEM and need something like MemoryHeapPmem, in this case this
should be implemented in device specific code (HAL).

This will most likely break older no longer supported targets.

Change-Id: I434e4291219950018de8b793b0403bb2d92dd5cc