Revert "[mmalloc] Use mremap to expand heaps (heap collision prevention)"

Revert "[mmalloc] Use mremap to expand heaps (heap collision prevention)"

This is really how is should be handled. However, this does not work
if someone has split the VMA in parts: this is done when creating
stacks on the heap without MC, mprotect() is used to create a guard
page for the stack.

This reverts commit ba3185d8a4f0a56804d3b1ff24e596096f06a37c.