Bluetooth: Audio: Remove BT_CODEC_MAX_DATA_LEN > 0 checks

The minimal value of BT_CODEC_MAX_DATA_LEN is 1, so it
is always > 0.

Includes to audio.h in shell bt.c was removed as audio.h
should only be included if BT_AUDIO=y.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>