monitor: Drop dead QMP check from monitor_read_password()

Function is only called in HMP context since commit 333a96ec "qapi:
Convert change".
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

monitor.c

@@ -266,10 +266,7 @@ void monitor_read_command(Monitor *mon, int show_prompt)
 int monitor_read_password(Monitor *mon, ReadLineFunc *readline_func,
                           void *opaque)
 {
-    if (monitor_ctrl_mode(mon)) {
-        qerror_report(QERR_MISSING_PARAMETER, "password");
-        return -EINVAL;
-    } else if (mon->rs) {
+    if (mon->rs) {
         readline_start(mon->rs, "Password: ", 1, readline_func, opaque);
         /* prompt is printed on return from the command handler */
         return 0;