qmp: Eliminate silly QERR_COMMAND_NOT_FOUND macro

The QERR_ macros are leftovers from the days of "rich" error objects.
They're used with error_set() and qerror_report(), and expand into the
first *two* arguments.  This trickiness has become pointless.  Clean
this one up.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>

include/qapi/qmp/qerror.h

@@ -52,9 +52,6 @@ void qerror_report_err(Error *err);
 #define QERR_BUS_NOT_FOUND \
     ERROR_CLASS_GENERIC_ERROR, "Bus '%s' not found"
 
-#define QERR_COMMAND_NOT_FOUND \
-    ERROR_CLASS_COMMAND_NOT_FOUND, "The command %s has not been found"
-
 #define QERR_DEVICE_ENCRYPTED \
     ERROR_CLASS_DEVICE_ENCRYPTED, "'%s' (%s) is encrypted"