CLEANUP: quic/receiver: remove the now unused tx_qring list

CLEANUP: quic/receiver: remove the now unused tx_qring list

The tx_qrings[] and tx_qring_list in the receiver are not used
anymore since commit f2476053f ("MINOR: quic: replace custom buf on Tx
by default struct buffer"), the only place where they're referenced
was in quic_alloc_tx_rings_listener(), which by the way implies that
these were not even freed on exit.

Let's just remove them. This should be backported to 2.6 since the
commit above also was.