dtc: Remove support for common.dts

Remove the common.dts file which has been used for a year.

common.dts at one point allowed us to conditionally add an MCUBoot
overlay based on Kconfig.

but since DT lost access to Kconfig options it has been unused.

The overridable variable DTS_COMMON_OVERLAYS, which by default points
to common.dts, is also unused in-tree, and any out-of-tree usage can
be ported over to use DTC_OVERLAY_FILE instead, so we remove the
variable as well.

This simplifies the configuration system.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>