Remove team-reviewers because it's failing

```
Error: Reviews may only be requested from collaborators. One or more of the
teams you specified is not a collaborator of the readthedocs/sphinx-hoverxref
repository.
```

It seems it's a permission issue. See peter-evans/create-pull-request#155 (comment)

However, we are not having this issue in .org. I'm removing the reviewers for
now just to move forward because we haven't updated the requirements in a while now.

Author: humitos

.github/workflows/pip-tools.yaml

@@ -38,4 +38,3 @@ jobs:
           delete-branch: true
           branch: dependencies/pip-tools
           branch-suffix: short-commit-hash
-          team-reviewers: readthedocs/backend