Remove unnecessary Windows-specific basebackup code.

Remove unnecessary Windows-specific basebackup code.

Commit c6f2f016 added an explicit check for a Windows "junction point".
That turned out to be needed only because get_dirent_type() was busted
on Windows.  It's been fixed by commit 9d3444dc, so remove it.

Add a TAP-test to demonstrate that in-place tablespaces are copied by
pg_basebackup.  This exercises the codepath that would fail before
c6f2f016 on Windows, and shows that it still doesn't fail now that we're
using get_dirent_type() on both Windows and Unix.

Back-patch to 15, where in-place tablespaces arrived and caused this
problem (ie directories where previously only symlinks were expected).

Reviewed-by: Michael Paquier 
Discussion: https://postgr.es/m/CA%2BhUKGLzLK4PUPx0_AwXEWXOYAejU%3D7XpxnYE55Y%2Be7hB2N3FA%40mail.gmail.com