Fix TerminalPage not being released on window close (#17107)

Because this holds onto the root element, `TerminalPage` gets "leaked"
on Windows 10 when a window is closed until another is opened.

## Validation Steps Performed
* Set a breakpoint in `Renderer::~Renderer`
* Open and close a window
* Breakpoint used to not get hit and now it does ✅

src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp

@@ -405,8 +405,6 @@ bool NonClientIslandWindow::Initialize()
 // - <none>
 void NonClientIslandWindow::SetContent(winrt::Windows::UI::Xaml::UIElement content)
 {
-    _clientContent = content;
-
     _rootGrid.Children().Append(content);
 
     // SetRow only works on FrameworkElement's, so cast it to a FWE before

src/cascadia/WindowsTerminal/NonClientIslandWindow.h

@@ -58,7 +58,6 @@ private:
     std::optional<til::point> _oldIslandPos;
 
     winrt::TerminalApp::TitlebarControl _titlebar{ nullptr };
-    winrt::Windows::UI::Xaml::UIElement _clientContent{ nullptr };
 
     wil::unique_hbrush _backgroundBrush;
     til::color _backgroundBrushColor;