Windows has this, and so do all other live platforms according to the
buildfarm, so remove the configure probe and src/port/ substitution.

This also lets us get rid of some configure probes that existed only
to support src/port/isinf.c.  I kept the port.h hack to force using
__builtin_isinf() on clang, though.

This is part of a series of commits to get rid of no-longer-relevant
configure checks and dead src/port/ code.  I'm committing them separately
to make it easier to back out individual changes if they prove less
portable than I expect.

Discussion: https://postgr.es/m/15379.1582221614@sss.pgh.pa.us