arm64: dts: qcom: sc7180: Delete mrbland

The mrbland board was never actually produced and there has been no
activity around the board for quite some time. It seems highly
unlikely to magically get revived. There should be nobody in need of
these device trees, so let's delete them. If somehow the project
resurrects itself then we can re-add support, perhaps just for -rev1+.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20230302131031.v2.4.I79eee3b8e9eb3086ae02760e97a2e12ffa8eb4f0@changeid