samples: Bluetooth: BAP: Unicast Server: Refactor TX

Refactors the TX of the unicast server. This commit effectively
copies the TX approach of the unicast client sample, to make
the sample more similar.
This changes the TX to be based on the number of complete
packets event (which results in free'ing the buffer), instead of
a timer. This completely ensures that we do not skip any sequence
numbers which may be rejected by the controller.

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