CLEANUP: xprt_quic: remove dead callbacks prepare_srv/destroy_srv

CLEANUP: xprt_quic: remove dead callbacks prepare_srv/destroy_srv

QUIC xprt defines prepare_srv callback. The main objective of this
callback is to setup server XPRT to QUIC.

This should be called once the server is fully configured, with related
code blocks in proxy_finalize() and cli_parse_add_server(). However,
this is in fact dead code as prepare_srv is in fact called through
XPRT_SSL. This is still functional though because SSL and QUIC share the
same prepare_srv, so this issue is not visible. This is the same
situation for destroy_srv callback.

This patch removes the dead code to ensure there is no ambiguity here.
There is still a design issue which should be fixed later as it's not
expected for QUIC code to rely on XPRT_SSL layer.