`dolt remote remove` leaves an upstream config behind

A branch can record an upstream, the branch that's merged into it by default, or that it's rebased onto. In Dolt, it's stored under `.dolt/repo_state.json` as two fields: the remote and a ref naming the branch on that remote. `dolt remote remove` clears the remote name but keeps the ref, making it ambiguous with a local branch upstream config.

```sh
$ dolt push -u origin b1:other
$ dolt remote remove origin
$ cat .dolt/repo_state.json
"branches": {
"b1": {
"head": "refs/heads/other",
"remote": ""
}
}

$ dolt branch --set-upstream-to other b1
branch 'b1' set up to track 'other'
$ cat .dolt/repo_state.json
"branches": {
"b1": {
"head": "refs/heads/other",
"remote": ""
}
}
```

When the upstream lives in the local repository, Git indicates it with a '.' character: [the relative path setting](https://git-scm.com/docs/git-config#Documentation/git-config.txt-branchnamemerge).

```sh
$ git config --get-regexp ^branch\.b1\.
branch.b1.remote origin
branch.b1.merge refs/heads/other
$ git remote remove origin
$ git config --get-regexp ^branch\.b1\.
(no output)

$ git branch --set-upstream-to=other2 b1
$ git config --get-regexp ^branch\.b1\.
branch.b1.remote .
branch.b1.merge refs/heads/other2
```

I've left a TODO in `UpstreamRef` since this doesn't allow us to discern if we want the ref.
https://github.com/dolthub/dolt/blob/92de41bbaef077eb6db53bce2b5d5f8ac001891f/go/libraries/doltcore/env/remotes.go#L811-L816

Start in go/libraries/doltcore/env/remotes.go at the UpstreamRef TODO, then trace the remote removal and branch upstream-setting entry points. Reproduce the commands in the issue and inspect .dolt/repo_state.json; done means remote removal no longer leaves an ambiguous ref, while local upstreams remain distinguishable.

Written by the indexing model from the issue text.