[PATCH][AArch64] Remove aarch64_frame_pointer_required

[Wilco Dijkstra]

Richard Sandiford wrote:
> But there's the third question of whether the frame pointer is available
> for general allocation.  By removing frame_pointer_required, we're saying
> that the frame pointer is always available for general use.  

Unlike on ARM/Thumb-2, the frame pointer is unfortunately never available for
general allocation on AArch64 - so we cannot use it for something actually useful.
A while back there were mid-end patches proposed to allow general allocation
of FP but those weren't accepted.

Wilco