We already test that postscript output supports SOURCE_DATE_EPOCH in
test_determinism. Removing the date in test_savefig_to_stringio in all
cases makes it easier to reuse the test in mplcairo, as cairo does not
support SOURCE_DATE_EPOCH (or manually setting the date in postscript
output in any case); the test is not about SOURCE_DATE_EPOCH support
anyways.

PR Summary

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and pydocstyle<4 and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).