dt-bindings: watchdog: Drop duplicate moxa,moxart-watchdog.txt

"moxa,moxart-watchdog" is already documented in faraday,ftwdt010.yaml,
so drop the old text binding.

Reviewed-by: Guenter Roeck <[e-mail]>
Fixes: 289660a4af0e ("dt-bindings: watchdog: convert faraday,ftwdt010 to yaml")
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring (Arm) <[e-mail]>

Documentation/devicetree/bindings/watchdog/moxa,moxart-watchdog.txt

···
-MOXA ART Watchdog timer
-
-Required properties:
-
-- compatible : Must be "moxa,moxart-watchdog"
-- reg : Should contain registers location and length
-- clocks : Should contain phandle for the clock that drives the counter
-
-Example:
-
-	watchdog: watchdog@98500000 {
-		compatible = "moxa,moxart-watchdog";
-		reg = <0x98500000 0x10>;
-		clocks = <&coreclk>;
-	};