Remove rd_variance_adjustment

This function is called after `super_block_yrd` and assumes that the dst
buffer is correct but that is no longer always the case after
daf841b4a10ece1b6831300d79f271d00f9d027b since we don't call
`txfm_rd_in_plane` after the RDO loop in `choose_tx_size_from_rd`.
We could fix this by always saving and restoring the dst buffer but
removing `rd_variance_adjustment` is a better solution:
- Getting the dst buffer always right is tricky as demonstrated by the
  fact that it is wrong now, even if we fix it now we could break it later
  and not notice
- Perceptual weighting is a good idea but `rd_variance_adjustment` is the
  wrong approach as it weights both the rate and the distortion:
  to get meaningful units you should only weight the distortion,
  weighting rate means that we pretend some bits cost less than other
  bits, this is not the case. The distortion weighting approach is
  implemented by Daala in `od_compute_dist` and we plan to experiment
  with this in AV1 too.
- Removing `rd_variance_adjustment` improves coding efficiency on all
  metrics, here are the results for objective-1-fast using the Low
  Latency settings:

      PSNR Y:     -0.14%
     PSNRHVS:     -0.17%
        SSIM:     -0.12%
      MSSSIM:     -0.12%
   CIEDE2000:     -0.07%

Change-Id: I74b26b568ee65f56521646b8f30dd53bcd29fce3