private void removeButton_Click(object sender, EventArgs e)
 {
     try
     {
         data.RemoveSubject(subject.Id);
         MessageBox.Show("Dėstomas dalykas pašalintas iš sistemos");
         foreach (Label l in this.Controls.OfType <Label>())
         {
             l.Visible = false;
         }
         removeButton.Visible = false;
     }
     catch (Exception exc)
     {
         MessageBox.Show(exc.Message);
     }
 }