Remove //third_party/avif/libavif

An empty directory was accidentally committed in 0f14062b...
as part of the dav1d roll. This CL cleans up the mistake.

Bug: 1010547
Change-Id: I29fd4793b817d64adf9832df67f7a82a6dfde1cf

TBR: sky@chromium.org
Change-Id: I29fd4793b817d64adf9832df67f7a82a6dfde1cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841666
Commit-Queue: Thomas Guilbert <tguilbert@chromium.org>
Auto-Submit: Thomas Guilbert <tguilbert@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704895}

libavif @ 22c1ad9f

-Subproject commit 22c1ad9f706ee00c3cdd0bb71ec4b7818de2f6c3