Revert "CLN: core/dtypes/cast.py::maybe_downcast_to_dtype"
Reverts pandas-dev/pandas#37050
opened to get mypy to green. other options available xref https://github.com/pandas-dev/pandas/pull/37050#issuecomment-708487786

cc @arw2019  failure unrelated

```
=========================== short test summary info ============================
FAILED pandas/tests/io/test_html.py::TestReadHtml::test_banklist_url_positional_match[bs4]
= 1 failed, 88193 passed, 1148 skipped, 1446 xfailed, 74 warnings in 1718.54s (0:28:38) =
```