CLN: remove unreachable code in pandas/core/groupby/generic.py::DataFrameGroupBy::_transform_general
Is this part of the code
```
        else:
            # Handle cases like BinGrouper
            return self._concat_objects(keys, values, not_indexed_same=not_indexed_same)
```
necessary? The condition before it is
```
elif self.grouper.groupings is not None
```
. However, even in the case of `BinGrouper`, I don't believe `BinGrouper.groupings` can be `None`:
```
>>> from pandas.core.groupby.ops import BinGrouper
>>> BinGrouper([], []).groupings                                                                                                                                                                   
[Grouping(None)]
```

 and so this code is unreachable. It's not covered by the tests, at least - https://codecov.io/gh/pandas-dev/pandas/src/master/pandas/core/groupby/generic.py#L1354

If it can be removed, I've taken it out and then shifted the code above it back by one tab (because we no longer need the guard)