Windows has this, and so do all other live platforms according to the
buildfarm; it's been required by POSIX since SUSv2.  So remove the
configure probe and tests of HAVE_WCHAR_H.

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