Move -ffast-math defense to float.c and remove the configure check.

Move -ffast-math defense to float.c and remove the configure check.

We had defenses against -ffast-math in timestamp-related files,
which is a pretty obsolete place for them since we've not supported
floating-point timestamps in a long time.  Remove those and instead
put one in float.c, which is still broken by using this switch.
Add some commentary to put more color on why it's a bad idea.

Also remove the check from configure.  That was just there to fail
faster, but it doesn't really seem necessary anymore, and besides
we have no corresponding check in meson.build.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Suggested-by: Andres Freund <andres@anarazel.de>
Suggested-by: Peter Eisentraut <peter@eisentraut.org>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/abFXfKC8zR0Oclon%40ip-10-97-1-34.eu-west-3.compute.internal