new clang warnings about unused global variables

The new clang 23 has a new warning apparently enabled by default (at
least by default in a PostgreSQL build) about set-but-unused static
global variables: -Wunused-but-set-global. This triggers a few warnings
in PostgreSQL code. See attached patches. The first four of these are
genuine hits; these variables were never read from or the last reading
use was removed some time ago. The last one in pg_ctl is about options
that can be set from the command line on any platform but are only used
on Windows. This seems kind of weird behavior; maybe in the future we
should make those options error out on non-Windows, and then the
variables wouldn't be uselessly set.