CLEANUP: mux-quic: remove loop on sending frames

CLEANUP: mux-quic: remove loop on sending frames

qc_send_app_pkts() has now a while loop implemented which allows to send
all possible frames even if the send buffer is full between packet
prepare and send. This is present since commit :
  dc07751ed7ebad10f49081d28a9a5ae785f53d76
  MINOR: quic: Send packets as much as possible from qc_send_app_pkts()

This means we can remove code from the MUX which implement this at the
upper layer. This is useful to simplify qc_send_frames() function.

As mentionned commit is subject to backport, this commit should be
backported as well to 2.6.