TST (string dtype): remove usage of arrow_string_storage fixture
This removes one other use of `"pyarrow_numpy"` in the tests.

This essentially reverts https://github.com/pandas-dev/pandas/pull/54535, because we now again can check `.storage == "pyarrow" for both dtype variants that use pyarrow as storage. 

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