Remove deprecated build scripts.

[jkoritzinsky]

No Merge. Part of batched rollout in May: #35202

Remove the deprecated CoreCLR build scripts (build.cmd/build.sh and build.proj).

We're removing these scripts because I keep catching people making infra changes only in these scripts and then have to point them to the scripts we use in CI so their changes are actually effective.

[safern]

Should we update any build defs that might be using these scripts?

[jkoritzinsky]

There should be no build definitions using these scripts, but if there are, they should be updated.

I've validated that the CoreCLR build and formatting jobs don't use these scripts, and the optimization team and source-build team have moved off of these scripts to my knowledge.

Tagging @dseefeld and @brianrob so they can let us know if they haven't moved off these scripts yet and won't be able to do so by May 4th.

[safern]

I asked cause I recalled seeing one, which just found, but I'm updating here: https://github.com/dotnet/runtime/pull/35011/files#diff-3672729464fb95f4293e8adfcbdbf3e6L11

[jkoritzinsky]

Right.... I totally forgot about the networking stress tests since they use a completely separate pipeline. Yeah we need to update those. I can go do that if you want.

[jkoritzinsky]

Actually, you're already fixing it, so that's covered then.

[brianrob]

I have moved optimization training off of these scripts. Thanks for adding me here @jkoritzinsky.