dts: drop HAS_DTS

HAS_DTS has become a redundant option. All Zephyr architectures now
select this option, meaning devicetree has become a de-facto
requirement.  In fact, if any board does not provide a devicetree
source, the build system uses an empty stub, meaning the devicetree
machinery always runs.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>