Setting OUTPUT_FORMAT through the CMake variable probably never
functioned and seems to be a remnant (possibly a typo in the
documentation) from the time when the -DOUTPUT_JSON=1 CMake variable was
used to specify the JSON output format, prior to the introduction of the
--format option. This removes the note regarding the option to choose
the output format using -DOUTPUT_FORMAT.
Closes https://github.com/espressif/esp-idf/issues/14383
Signed-off-by: Frantisek Hrbata <frantisek.hrbata@espressif.com>