adc: Remove CONFIG_ADC_MCUX_ETC

This Kconfig does not belong in the ADC folder, because there is not
actually a zephyr ADC driver for this. Also, remove HAS_MCUX_ADC_ETC as
well because it is a useless config.

The cmake line to pull in this driver from the SDK in the zephyr repo is
totally unnecessary. If a user wants to use this SDK driver they can add
it to their build like any other SDK driver or any other
external code module. Zephyr should not be a cesspool of random build
glue for random pieces of code.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>