Pybind11 cleanup - #28857

PR summary

As a followup to #28856, we can now do some final cleanup:

• drop all the old Python → C converters, and other helpers
• also move py_converters_11.{cpp,h} into py_converters.{cpp,h}
• remove NumPy as a build dependency

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• [n/a] Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines