dt-bindings: touchscreen: remove touchscreen.txt

With commit 1d6204e ("dt-bindings: touchscreen: Add touchscreen
schema") touchscreen.txt is no longer needed, and since no other file
refers to it, it can be safely removed.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250925153144.4082786-3-dario.binacchi@amarulasolutions.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Author: passgat