qerror: QERR_IO_ERROR is no longer used, drop

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240513141703.549874-7-armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>

include/qapi/qmp/qerror.h

@@ -20,9 +20,6 @@
 #define QERR_INVALID_PARAMETER_VALUE \
     "Parameter '%s' expects %s"
 
-#define QERR_IO_ERROR \
-    "An IO error has occurred"
-
 #define QERR_MISSING_PARAMETER \
     "Parameter '%s' is missing"