Back off chattiness in RemovePgTempFiles().

Back off chattiness in RemovePgTempFiles().

In commit 561885db0, as part of normalizing RemovePgTempFiles's error
handling, I removed its behavior of silently ignoring ENOENT failures
during directory opens.  Thomas Munro points out that this is a bad idea at
the top level, because we don't create pgsql_tmp directories until needed.
Thus this coding could produce LOG messages in perfectly normal situations,
which isn't what I intended.  Restore the suppression of ENOENT logging,
but only at top level --- it would still be unexpected for a nested temp
directory to disappear between seeing it in the parent directory and
opening it.

Discussion: https://postgr.es/m/CAEepm=2y06SehAkTnd5sU_eVqdv5P-=Srt1y5vYNQk6yVDVaPw@mail.gmail.com