short-circuit av1_get_tx_scale to always return zero when
CONFIG_DAALA_TX, and remove it from the actual Daala TX toplevel

This has potential overflow consequences for any metrics computation
based on pixels; as such, also force use of the high-bitdepth path in
each of these case.

subset-1:
monty-rest-of-stack-baseline-s1@2017-11-13T00:39:03.881Z ->
monty-rest-of-stack-noshift-s1@2017-11-13T14:37:42.541Z

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0030 | -0.0523 |  0.2656 |  -0.0239 | -0.0033 | -0.0029 |     0.0067

objective-1-fast --limit=4:
monty-rest-of-stack-baseline-o1f4@2017-11-13T00:37:06.999Z ->
monty-rest-of-stack-noshift-o1f4@2017-11-13T14:37:16.992Z

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0264 |  0.2303 |  0.0822 |  -0.0109 | -0.0395 | -0.0709 |     0.0538

Change-Id: I57da71861f105dc7a404fa75a75bde573855ef79