dt-bindings: phy: Remove phy-stih41x-usb binding

The driver was removed in 2016 in commit fb954c48 ("phy:
stih41x-usb: Remove usb phy driver and dt binding  documentation.") and
somehow the DT binding got dropped despite the subject.

Cc: Peter Griffin <peter.griffin@linaro.org>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Link: https://lore.kernel.org/r/20200826184851.3431531-2-robh@kernel.org


Signed-off-by: Rob Herring <robh@kernel.org>