boards: nrf9160dk: Remove empty .conf files

After 3a58b45ced53ce4c601c3c2ac8b7c2f1d5a57aeb introduced the option
of specifying valid board revisions, it is no longer needed to provide
a .conf file for each revision. Remove then the files that were added
solely for this reason and use the VALID_REVISIONS parameter instead.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
8 files changed