Remove redundant feedback_packet_seq_num_set_ in RtpVideoSender

The state this set tracks (whether this is new feedback for a packet
belonging to a media ssrc) can already be inferred from data provided
by the SendTimeHistory: if packet send time is not populated in the
feedback it's either because:
1. The feedback has already been processed
2. The receiver is sending feedback for bogus non-existent packets

If the first case, this maps to |feedback_packet_seq_num_set_|
containing the packet, if the ssrc (present in the feedback) is part
of the media ssrcs.

In the second case, this data should be ignored anyway.

Bug: webrtc:10604
Change-Id: If4828091142d68baa8dbb62be9d0b24ccaaa9546
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135163
Commit-Queue: Erik Språng <sprang@webrtc.org>
Reviewed-by: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27882}