Clean up the backups package.

This is intended as an idempotent change in preparation for further patches.
Rather than hardcoding this path here, can you please change this to:

```go
import "github.com/juju/juju/mongo"
...
mongod, err := mongo.Path()
if err != nil {
    return "", err
}
mongoDumpPath := mongod + "ump"
...
```