Remove PRIORITY frame. PRIORITY frame has been removed from the draft HTTP/3 spec at https://github.com/quicwg/base-drafts/pull/2922. Also remove protocol flag FLAGS_quic_allow_http3_priority. gfe-relnote: n/a, change to QUIC v99-only code. Protected by existing disabled gfe2_reloadable_flag_quic_enable_version_99. PiperOrigin-RevId: 289115003 Change-Id: I52891171f4c2b11107f0afc2db6e069963b13d6e