Strip import statement if all imports are removed.

Currently, given:

```js
import { SOME_FLAG } from 'blah/features';

if (SOME_FLAG) {
console.log('whatever');
}
```

With this config:

```js
{
plugins: [
['babel-debug-macros', {
features: {
name: 'blah',
source: 'blah/features',
flags: { SOME_FLAG: true }
},
}]
]
}
```

We remove the usage of `SOME_FLAG`, but leave the import statement. This forces us to also emit a `blah/features` module even if we never have runtime enableable features.

IMO, we should remove imports if they are no longer used...