Remove extraneous build files

[christycylee]

mold uses cmake, the BUCK files from zstd are not needed.

[rui314]

I just copied the entire source tree of zstd to our third-party directory. All of their files may not be needed, but still we can keep them, can't we?

[christycylee]

The BUCK files are malformed which breaks repos that use buck builds by default. I'm also happy to fix the BUCK files if that's better.

[rui314]

Let's remove them for now. Do you mind if I ask you to fix it in upstream (https://github.com/facebook/zstd) so that the issue is resolved completely?

[christycylee]

Do you mind if I ask you to fix it in upstream

For sure!

[rui314]

I'm not sure what these build files are for, but if they are malformed and cannot be used, you may want to remove them from upstream instead of fixing them. If they are unusable, these files have no users.

For tracking purpose, refer to this PR when you make a new PR for the upstream.