This reverts commit e6582161.

Remove the DXT1->DXT3 transcoding but leave in the MacOS workarounds for fixing
the alpha channel of DXT1 textures.

BUG=angleproject:3729

Change-Id: Ib276c1e4a58155866da8c661cba2063a4e4304fe
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1846015Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>