revert vtkChartXY downcast fix

This reverts a subset the changes of !4960 (merged) inside vtkSMContextViewProxy::OnLeftButtonReleaseEvent

Else I get a segfault at the selection on a parallel coordinates plot if I update from paraview 5.9 to 5.10 with the following stack:

0x7f2cf2b900b3 __libc_start_main + 243
32      0x56130b02d0ac /tmp/.mount_pvwidgBWQh0Y/usr/bin/micropv(+0x190ac) [0x56130b02d0ac]
31      0x7f2cf470c246 QCoreApplication::exec() + 150
30      0x7f2cf47044db QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 299
29      0x7f2cf475d565 QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 101
28      0x7f2cef0724a3 g_main_context_iteration + 51
27      0x7f2cef072400 /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0(+0x52400) [0x7f2cef072400]
26      0x7f2cef07217d g_main_context_dispatch + 637
25      0x7f2ce6a3132e /tmp/.mount_pvwidgBWQh0Y/usr/lib/libQt5XcbQpa.so.5(+0x7932e) [0x7f2ce6a3132e]
24      0x7f2cf4aca35b QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 187
23      0x7f2cf4af010b QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent*) + 603
22      0x7f2cf4aee7d3 QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent*) + 1763
21      0x7f2cf470593a QCoreApplication::notifyInternal2(QObject*, QEvent*) + 394
20      0x7f2cf51290f0 QApplication::notify(QObject*, QEvent*) + 816
19      0x7f2cf511fa66 QApplicationPrivate::notify_helper(QObject*, QEvent*) + 134
18      0x7f2cf51811ec /tmp/.mount_pvwidgBWQh0Y/usr/lib/libQt5Widgets.so.5(+0x1cc1ec) [0x7f2cf51811ec]
17      0x7f2cf517e35d /tmp/.mount_pvwidgBWQh0Y/usr/lib/libQt5Widgets.so.5(+0x1c935d) [0x7f2cf517e35d]
16      0x7f2cf5128457 QApplicationPrivate::sendMouseEvent(QWidget*, QMouseEvent*, QWidget*, QWidget*, QWidget**, QPointer<QWidget>&, bool, bool) + 439
15      0x7f2cf470593a QCoreApplication::notifyInternal2(QObject*, QEvent*) + 394
14      0x7f2cf5129343 QApplication::notify(QObject*, QEvent*) + 1411
13      0x7f2cf511fa66 QApplicationPrivate::notify_helper(QObject*, QEvent*) + 134
12      0x7f2cf568165f QVTKOpenGLNativeWidget::event(QEvent*) + 31
11      0x7f2cf567ea5d QVTKInteractorAdapter::ProcessEvent(QEvent*, vtkRenderWindowInteractor*) + 2893
10      0x7f2cf3489aa2 /tmp/.mount_pvwidgBWQh0Y/usr/lib/libvtkCommonCore-pv5.10.so.1(+0x4ffaa2) [0x7f2cf3489aa2]
9       0x7f2cf336a54d vtkCallbackCommand::Execute(vtkObject*, unsigned long, void*) + 29
8       0x7f2cf10873d8 vtkInteractorStyle::ProcessEvents(vtkObject*, unsigned long, void*, void*) + 1016
7       0x7f2cf42a93bd vtkPVContextInteractorStyle::OnLeftButtonUp() + 13
6       0x7f2cf072f6f8 vtkContextInteractorStyle::OnLeftButtonUp() + 152
5       0x7f2cf3d33384 vtkContextScene::ButtonReleaseEvent(vtkContextMouseEvent const&) + 468
4       0x7f2cf3489944 /tmp/.mount_pvwidgBWQh0Y/usr/lib/libvtkCommonCore-pv5.10.so.1(+0x4ff944) [0x7f2cf3489944]
3       0x7f2cf432b89f vtkObject::vtkClassMemberCallback<vtkSMContextViewProxy>::operator()(vtkObject*, unsigned long, void*) + 63
2       0x7f2cf432a884 vtkSMContextViewProxy::OnLeftButtonReleaseEvent() + 164
1       0x7f2cf079cca4 vtkChartLegend::GetPointVector() + 4
0       0x7f2cf2baf210 /lib/x86_64-linux-gnu/libc.so.6(+0x46210) [0x7f2cf2baf210]

I have no clue what I'm doing here, does this make sense @mwestphal ?