dt-bindings: Drop empty and unreferenced binding .txt files

Drop a couple of old, empty .txt binding files which are no longer
referenced.
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220422191958.2589318-1-robh@kernel.org

Documentation/devicetree/bindings/eeprom/at24.txt

-This file has been moved to at24.yaml.

Documentation/devicetree/bindings/mtd/common.txt

-This file has been moved to mtd.yaml.

Documentation/devicetree/bindings/rtc/rtc.txt

-This file has been moved to rtc.yaml.