MINOR: quic: remove connection arg from qc_new_conn()

MINOR: quic: remove connection arg from qc_new_conn()

This patch is similar to the previous one, this time dealing with
qc_new_conn(). This function was asymetric on frontend and backend side,
as connection argument was set only in the latter case.

This was required prior due to qc_alloc_ssl_sock_ctx() signature. This
has changed with the previous patch, thus qc_new_conn() can also be
realigned on both FE and BE sides. <conn> member of quic_conn instance
is always set outside it, in qc_xprt_start() on the backend case.