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>

Documentation/devicetree/bindings/arm/insignal-boards.txt

-* Insignal's Exynos4210 based Origen evaluation board
-
-Origen low-cost evaluation board is based on Samsung's Exynos4210 SoC.
-
-Required root node properties:
-    - compatible = should be one or more of the following.
-        (a) "samsung,smdkv310" - for Samsung's SMDKV310 eval board.
-        (b) "samsung,exynos4210"  - for boards based on Exynos4210 SoC.