cmake: dts: removing generated <board>.dts_compiled

The file <board>.dts_compiled was made obsolete in 8d8b0697.

<board>.dts_compiled contains only a reference to zephyr.dts.

Users interested in devicetree should be familiar with zephyr.dts,
therefore it's time to remove the unused <board>.dts_compiled.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>