String dtype: restrict options.mode.string_storage to python|pyarrow (remove pyarrow_numpy)
We allowed to set `"pyarrow_numpy"` as a value to the `mode.string_storage` option.

I assume this was mostly done for testing purposes, because we explicitly documented that this option would be ignored when enabling the `future.infer_string` option (i.e. when "pyarrow_numpy" is being used by default).

In context of the new string dtype (PDEP-14), I think we should remove that option again (since we will generally deprecate/remove "pyarrow_numpy"). Given the above (not documented), I opted for directly removing it from the option instead of first deprecating it (for the StringDtype constructor / string alias, we will first deprecate it). But I did add a clarification to the option error message in case someone actually did use it.

xref https://github.com/pandas-dev/pandas/issues/54792
Thanks @jorisvandenbossche 