Revert "graceful shutdown" changes for Windows.

Revert "graceful shutdown" changes for Windows.

This reverts commits 6051857fc and ed52c3707 in HEAD (they were already
reverted in the back branches).  Further testing has shown that while
those changes do fix some things, they also break others; in particular,
it looks like walreceivers fail to detect walsender-initiated connection
close reliably if the walsender shuts down this way.  A proper fix for
this seems possible but won't be done in time for v15.

Discussion: https://postgr.es/m/CA+hUKG+OeoETZQ=Qw5Ub5h3tmwQhBmDA=nuNO3KG=zWfUypFAw@mail.gmail.com
Discussion: https://postgr.es/m/CA+hUKGKkp2XkvSe9nG+bsgkXVKCdTeGSa_TR0Qx1jafc_oqCVA@mail.gmail.com