Assume that we have cbrt().
Windows has this, and so do all other live platforms according to the
buildfarm, so remove the configure probe and float.c's substitute code.
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