bluetooth: controller: nrf: Clean up a bit radio_nrf5_ppi.h

Remove no longer needed definition of NRFX_PPI_CHANNELS_USED_BY_PWM_SW
(after recent changes in the nrf_hw_models models, also for simulated
nRF boards the nrfx_glue.h file is processed, so this symbol is always
defined).
Remove also a couple of *_PPI_x_INCLUDE macros, to make the code a bit
clearer and to avoid things like a build assertion (also removed here)
that checks if HAL_SW_SWITCH_GROUP_TASK_DISABLE_PPI_0_INCLUDE is indeed
defined as BIT(HAL_SW_SWITCH_GROUP_TASK_DISABLE_PPI(0)).

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