MIPS: remove alloc_pci_controller prototype

Commit 610019baddcb4c4c323c12cd44ca7f73d7145d6f ("[MIPS] Remove unused
function alloc_pci_controller.") removed the function, but left the
prototype in the header file.

Remove it as well so people don't get tempted to use it and wonder why
it doesn't work.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5473/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>