drivers: adc_nrfx_saadc: Remove dead and misleading code

For a long time (since version 3.3.0) nrfx contained an incorrectly
defined symbol NRF_SAADC_8BIT_SAMPLE_WIDTH that was set to 8 for nRF54L
and nRF54H Series SoCs, which was probably only true for very early
engineering revisions of those. Based on this, the adc_nrfx_saadc driver
was incorrectly writing consecutive 8-bit samples in supplied buffers,
cutting off the highest 8 bits of the results. And for sequences with
multiple channels, it was even causing that the results written as
16-bit words by hardware were partially overwritten in next iteration.

In nrfx 3.12.0 (see commit f46798fa)
this was finally corrected - the symbol is now deprecated and it is
always set to 16. This commit is a follow-up to the above and removes
parts of adc_nrfx_saadc that now became dead code to prevent further
confusion regarding 8-bit sampling.

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