Assume that we have rint().

Assume that we have rint().

Windows has this since _MSC_VER >= 1200, and so do all other live
platforms according to the buildfarm, so remove the configure probe
and src/port/ substitution.

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