Remove deprecated *model.Client methods (#18042)

```release-note
Removed CheckUserMfa
Removed GetServerBusyExpires
```

api4/system_test.go

@@ -676,24 +676,6 @@ func TestGetServerBusy(t *testing.T) {
 	}, "as system admin")
 }
 
-func TestGetServerBusyExpires(t *testing.T) {
-	th := Setup(t)
-	defer th.TearDown()
-
-	th.App.Srv().Busy.Set(time.Second * 30)
-
-	t.Run("as system user", func(t *testing.T) {
-		_, resp := th.Client.GetServerBusyExpires()
-		CheckForbiddenStatus(t, resp)
-	})
-
-	th.TestForSystemAdminAndLocal(t, func(t *testing.T, c *model.Client4) {
-		expires, resp := c.GetServerBusyExpires()
-		CheckNoError(t, resp)
-		require.Greater(t, expires.Unix(), time.Now().Unix())
-	}, "as system admin")
-}
-
 func TestServerBusy503(t *testing.T) {
 	th := Setup(t)
 	defer th.TearDown()