powerpc/64: Remove maple platform

The maple platform was added in 2004 [1], to support the "Maple" 970FX
evaluation board.

It was later used for IBM JS20/JS21 machines, as well as the Bimini
machine, aka "Yellow Dog Powerstation".

Sadly all those machines have passed into memory, and there's been no
evidence for years that anyone is still using any of them.

Remove the platform and related code. It can always be reinstated if
there's interest.

Note that this has no impact on support for 970FX based Power Macs.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/mpe/linux-fullhistory.git/commit/?id=f0d068d65c5e555ffcfbc189de32598f6f00770c



Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patch.msgid.link/20241013102957.548291-1-mpe@ellerman.id.au