Drop widget invalidation in LinkGraphOverlay mask methods

Drop widget invalidation in LinkGraphOverlay mask methods

LinkGraphOverlay::SetCargoMask and LinkGraphOverlay::SetCompanyMask
are only called from LinkGraphLegendWindow::OnClick and from the
smallmap, but all calls from the smallmap code are followed by an
unconditional invalidation of the whole smallmap window, so only
LinkGraphLegendWindow::OnClick needs the widget invalidation done in
LinkGraphOverlay::SetCargoMask and LinkGraphOverlay::SetCompanyMask,
and then we know that the widget to invalidate is the viewport in
the main window.