Remove deprecated monitor_point C++ class

[oskooi]

Closes #3237.

The C++ class monitor_point (src/meep.hpp:1100-1128) is a legacy field-sampling helper that has long been documented as deprecated in doc/docs/C++_Developer_Information.md:83. It is a thin caching/bookkeeping wrapper: fields::get_point() already populates it by calling fields::get_field() internally (src/monitor.cpp:62). The documented replacement is to call fields::get_field() directly and, where spectral analysis is needed, feed a collected array into do_harminv().

monitor_point is C++-only — the Python and Scheme front-ends never use it (Python field sampling already goes through fields::get_field via fields::get_field_point, and Python harminv uses the standalone do_harminv wrapper). It is exposed to SWIG only incidentally via %include "meep.hpp", with no explicit typemaps, so removing it does not affect the bindings.

This PR deletes the class and its supporting fields methods, migrates the C++ unit tests that use it to fields::get_field (+ do_harminv where needed), and updates the docs.

[stevengj]

Also get rid of the corresponding configure.ac tests if (for now) we have nothing that uses FFTs.

[oskooi]

FFTW is still used — by src/casimir.cpp (the make_casimir_g function uses fftw_plan_dft_1d/fftw_execute and the FFTW2 paths). The configure.ac comment at line 146 even names both files.

The suggested cleanup is to update the stale comment in configure.ac to drop the src/monitor.cpp reference, since that file no longer uses FFTW.