Remove quic_bbr2_sender. It's no longer useful now that BBR2 code is in Chromium. gfe-relnote: n/a (no functional change) Merge note: please remove quic_bbr2_sender_impl.h from Chromium PiperOrigin-RevId: 291838586 Change-Id: I970301efae639fedb3efb93b7905702573fb1c47