Streamline `mk_arcade_joystick_rpi` versions

As confirmed by @ian57, we can now use the branch v0.1.6-hotkey-custom-gpio of recalbox/mk_arcade_joystick_rpi both when we want hotkeys or not.

In order to do this, we need to:

• tag the branch head as v0.1.6, so that it can be used by Buildroot (it does not support using branches directly)
• change mk_arcade_joystick_rpi.mk to always use MK_ARCADE_JOYSTICK_RPI_VERSION = v0.1.6
• remove BR2_PACKAGE_MK_ARCADE_JOYSTICK_RPI_HOTKEY from Config.in (🎁 bonus: there's also an unused BR2_PACKAGE_MK_ARCADE_JOYSTICK_RPI_I2C variable there 😏 )