adc: remove Kconfig.defconfig setting of ADC drivers

Now that ADC drivers are enabled based on devicetree
we need to remove any cases of them getting enabled by
Kconfig.defconfig* files as this can lead to errors.

Typically the Kconfig.defconfig* will blindly enable a
sensor and not respect the devicetree state of the ADC.
Additionally we can get problems with prj.conf/defconfig
getting incorrectly overridden.

Signed-off-by: Kumar Gala <galak@kernel.org>