opus: remove unused WebRtcOpus_PacketHasVoiceActivity

The side channel check was also wrong:

  if (channels == 2 &&
      (frame_data[frame][0] << (silk_frames + 1)) >> (8 - silk_frames))

The byte promotes to int, so the left shift does not discard the mid
channel flags and the right shift brings them back down. That made the
side channel VAD test fire on the mid channel LBRR flag, a false
positive on stereo SILK frames carrying in-band FEC.

The correct form truncates to 8 bits after the left shift:

  const uint8_t flags = frame_data[frame][0];
  const uint8_t side = static_cast<uint8_t>(flags << (silk_frames + 1));
  if (channels == 2 && side >> (8 - silk_frames))

Note: the encoder never produces this combination so no consequence.

Bug: None
Change-Id: I8b7118a0c749287e6850e3b8d3fb136e4d4f9809
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/501701
Reviewed-by: Jakob Ivarsson‎ <jakobi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Jakob Ivarsson‎ <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48557}