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...

No file or test is named in the issue. Start by locating the babel-debug-macros transform that processes imports from the configured feature source, then reproduce the example with SOME_FLAG removed. Done means the generated output no longer retains the import when all imported flags are removed, while the existing feature-removal behavior remains intact.

Written by the indexing model from the issue text.