qga: Drop dangling reference to QERR_QGA_LOGGING_DISABLED

qga: Drop dangling reference to QERR_QGA_LOGGING_DISABLED

slog()'s function comment advises to use QERR_QGA_LOGGING_DISABLED.
This macro never existed.  The reference got added in commit
e3d4d25206a "guest agent: add guest agent RPCs/commands" along with
QERR_QGA_LOGGING_FAILED, so maybe that one was meant.  However,
QERR_QGA_LOGGING_FAILED was never actually used, and was removed in
commit d73f0beadb5 "qerror.h: Remove unused error classes".

Drop the dangling reference.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20230207075115.1525-9-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>