tests: drivers: adc: Remove redundant logging configurations

This commit removes the redundant `LOG` and `LOG_IMMEDIATE` selections
in `prj.conf`, since the `TEST_LOGGING_DEFAULTS`, which is enabled by
default for all tests, selects `LOG` and `LOG_MINIMAL`.

This effectively allows the test to run with the default test logging
policy, set by the testing subsystem.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>