modules: hal_nordic: nrfx_glue: Remove NRFX_GPIOTE_CHANNELS_USED

This definition is no longer supported by the nrfx_gpiote driver (since
nrfx 3.2.0, reserved channels need to be defined per GPIOTE instance),
so remove it to not mislead users.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>