Get rid of define IVAS_MAX_NUM_CH

There is defined in the framework the macro

#define IVAS_MAX_NUM_CH 16 /* == MAX_OUTPUT_CHANNELS */

which is used sparsely and can be simply replaced by existing macros.

I thus propose to replace IVAS_MAX_NUM_CH

• by MAX_INPUT_CHANNELS at the encoder
• by MAX_OUTPUT_CHANNELS at the decoder

Please comment if you see it differently!