CLN remove unreachable code in pandas/core/groupby/generic.py::DataFrameGroupBy::_wrap_applied_output
NOTE: I'd originally [opened a PR](https://github.com/pandas-dev/pandas/pull/32583) to address this, but am now busy with other obligations + other PRs to respond to. So I'm opening it up as a good first issue for now, and will return to work on it if nobody takes it.

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In `pandas/core/groupby/generic.py::DataFrameGroupBy::_wrap_applied_output`, it seems this branch is never reached:

```
        else:
            # Handle cases like BinGrouper
            return self._concat_objects(keys, values, not_indexed_same=not_indexed_same)
```

Two things need to be done here:

1. refactor `_wrap_applied_output` by making a helper function out of some of its internals (it's currently very long, and changes are hard to review)
2. Remove this unused `else` branch