Bluetooth: controller: remove hidden GPIOTE configs

The GPIOTE channels used to manage radio front end module GPIOs are
configured with invisible Kconfig symbols. They are therefore not user
visible. Remove them by redefining them in terms of equivalent macros
defined in radio_nrf5.h. This lets us get rid of a couple of ifdefs
that are no longer needed as well.

This is because these invisible symbols depend on the
CONFIG_BT_CTLR_GPIO_* pin configuration symbols. We would like to move
all of that pin configuration to devicetree, because it's hardware
description. It is therefore inconvenient to have the GPIOTE channel
configuration set via Kconfig in a way that depends on something we
would like to remove.

No functional changes or user impact expected: invisible symbols
cannot be set in .conf files.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>