remove: contract nodes automatically

Remove contract certain nodes automatically on removal.
Example:

```
crane = {
   url = "github:ipetkov/crane";
   inputs.nixpkgs.follows = "nixpkgs";
   };
```
to
```
crane = {
  url = "github:ipetkov/crane";
};
```
should rather be:
```
crane.url = "github:ipetkov/crane";
```
in most cases.