dt-bindings: chosen: remove old .txt binding

chosen.txt has been replaced by a schema in dtschema[1] and is now out
of date as well. Remove it to avoid confusion.

[1] https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/chosen.yaml

Link: https://lore.kernel.org/lkml/c8dddfe6-6385-ed34-e789-9f845c8a32bd@linaro.org/
Link: https://lore.kernel.org/lkml/CAL_Jsq+uSdk9YNbUW35yjN3q8-3FDobrxHmBpy=4RKmCfnB0KQ@mail.gmail.com/


Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
[robh: Improve commmit msg]
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220628153354.870543-1-Jason@zx2c4.com