dt-bindings: arm: Remove obsolete insignal-boards.txt

The compatibles mentioned in insignal-boards.txt are already documented
under devicetree/bindings/arm/samsung/samsung-boards.txt.  Also the
contents of insignal-boards.txt is not accurate, e.g. does not mention
Arndale boards.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>