MINOR: quic: Useless rate sample member initialization

MINOR: quic: Useless rate sample member initialization

This poor/inefficient code has been revealed by coverity GH issue in #2788 where
some quic_cc_rs struct member initializations were mentionned as overwritten
(after initialization) before being used as follows:

CID 1565821:  Code maintainability issues  (UNUSED_VALUE)
/src/quic_cc_bbr.c: 1373 in bbr_handle_lost_packet()
1367     }
1368
1369     static void bbr_handle_lost_packet(struct bbr *bbr, struct quic_cc_path *p,
1370                                        struct quic_tx_packet *pkt,
1371                                        uint32_t lost)
1372     {
>>>     CID 1565821:  Code maintainability issues  (UNUSED_VALUE)
>>>     Assigning value "0UL" to "rs.tx_in_flight" here, but that stored value is overwritten before it can be used.
1373            struct quic_cc_rs rs = {0};
1374
1375            /* C.delivered = bbr->drs.delivered */
1376            bbr_note_loss(bbr, bbr->drs.delivered);
1377            if (!bbr->bw_probe_samples)
1378                    return; /* not a packet sent while probing bandwidth */

Remove the {0} initializer for <rs> variable. This is safe because the members
initializations of <rs> local variable passed to functions from
bbr_handle_lost_packet() are done. Add a comment to mention this.