tests: drivers: adc: move ad5592-adc node into native_posix.overlay

Remove the `app.overlay` file which has been
wrongly added in ad3c5a27 commit. Instead of that
add the ad5592-adc node into `native_posix.overlay` file.

Signed-off-by: Bartosz Bilas <b.bilas@grinn-global.com>