boards: nrf52840_pca10056: move modem configurtion to overlay

The wncm14a2a modem is an add-on/shield and not part of the
nrf52840_pca10056 board and should be maintained and configured
outside of the board.

This needs to be moved to a shield, see #10965.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>