MAINT: Remove FPE helper code that is unnecessary on C99/C++11
MAINT: Remove FPE helper code that is unnecessary on C99/C++11

C99 and C++11 standardize `fenv.h`, so we should not need the manual
definitions for these functions for platforms that do not support it.
See e.g.:  https://en.cppreference.com/w/c/numeric/fenv

If issues come up, the code can be restored.

This applies gh-21421 again, since we had to revert using the cpp file
and the change thus got lost.

mhvk/numpy,anntzer/numpy,endolith/numpy,pdebuyl/numpy,pdebuyl/numpy,pdebuyl/numpy,mhvk/numpy,mhvk/numpy,numpy/numpy,rgommers/numpy,endolith/numpy,mattip/numpy,charris/numpy,mhvk/numpy,anntzer/numpy,numpy/numpy,anntzer/numpy,rgommers/numpy,numpy/numpy,endolith/numpy,charris/numpy,anntzer/numpy,endolith/numpy,rgommers/numpy,mattip/numpy,charris/numpy,charris/numpy,mattip/numpy,mattip/numpy,mhvk/numpy,numpy/numpy,pdebuyl/numpy,rgommers/numpy