fix(taranis): remove unused function

[fpenard]

Summary of changes:

read_i2c_volume static function is defined but not used in targets/taranis/volume_i2c.cpp.

This one generates the next warning on build.

edgetx/radio/src/targets/taranis/volume_i2c.cpp:36:16: warning: 'int32_t read_i2c_volume()' defined but not used [-Wunused-function]
   36 | static int32_t read_i2c_volume()

[pfeerick]

@raphaelcoeffic Do we remove this, or revert #5863 and probably add && defined(AUDIO) so that debug building doesn't break? At present we can set, but not read the volume level on non-soft volume radios via cli.