cleanup: remove unused FEEDBACK_RING_SIZE define
cleanup: remove unused FEEDBACK_RING_SIZE define

The FEEDBACK_RING_SIZE define was left behind. It was part of the code
that handled the internal storage of feedbacks before the new API was
introduced.

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc