MINOR: quic: remove unneeded QUIC specific stopping function

MINOR: quic: remove unneeded QUIC specific stopping function

On CONNECTION_CLOSE reception/emission, QUIC connections enter CLOSING
state. At this stage, only CONNECTION_CLOSE can be reemitted and all
other exchanges are stopped.

Previously, on haproxy process stopping, if all QUIC connections were in
CLOSING state, they were released before their closing timer expiration
to not block the process shutdown. However, since a recent commit, the
closing timer has been shorten to a more reasonable delay. It is now
consider viable to respect connections closing state even on process
shutdown. As such, stopping specific code in QUIC connections idle timer
task was removed.

A specific function quic_handle_stopping() was implemented to notify
QUIC connections on shutdown from main() function. It should have been
deleted along the removal in QUIC idle timer task. This patch just does
this.