dt-bindings: power: supply: Drop power_supply.txt

The file has been replaced by power-supply.yaml and all links
have been updated to the new file.
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/power/supply/power_supply.txt

-This binding has been converted to yaml please see power-supply.yaml in this
-directory.