CLN/INT: remove usage of _internal_get_values
PR https://github.com/pandas-dev/pandas/pull/26409 introduced a `_internal_get_values` method for DataFrame, Series, Index, SparseArray and Categorical (in order to deprecate the public `get_values`). 

There are some remaining cases where `_internal_get_values` is used internally. Eventually, we should try to remove all those cases. For each of those usages, it will need to be inspected what object is actually being handled to choose the appropriate alternative.