samples: net: cellular_modem: Remove nRF9160DK examples

Remove nRF9160DK examples from Zephyr's cellular modem sample as
these use nRF91 as an external modem and the pin configuration
heavily depends on the build target.

Therefore we would like to host and document our own examples
within our own repository with the Serial Modem application.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>