Remove the Accessibility Tab completely from the app (#79)

See
https://github.com/OpenVR-Advanced-Settings/OpenVR-AdvancedSettings/issues/27

The Accessibility tab was made redundant with the implementation of the
new playspace/room/universe tab in
https://github.com/OpenVR-Advanced-Settings/OpenVR-AdvancedSettings/issues/60

The tab inherited from the PttController class, which required changes
in order to work with the new IVRInput system. See
https://github.com/OpenVR-Advanced-Settings/OpenVR-AdvancedSettings/issues/51
for more about the IVRInput system.

Removing the page now at an early state is going to make it easier to
refactor the PttController since only one class is inheriting it,
instead of two. Since the Accessibility tab is going to be removed
anyway, it makes sense to remove it now.

src/overlaycontroller.h

@@ -37,7 +37,6 @@
 #include "tabcontrollers/SettingsTabController.h"
 #include "tabcontrollers/ReviveTabController.h"
 #include "tabcontrollers/UtilitiesTabController.h"
-#include "tabcontrollers/AccessibilityTabController.h"
 
 #include "ivrinput/ivrinput.h"
 
@@ -50,7 +49,6 @@ namespace advsettings
 // Actual rates of updates are counter * vsync (~11ms)
 // Values chosen based on update speed priority
 // Avoid setting values to the same numbers.
-constexpr int k_accessibilitySettingsUpdateCounter = 151;
 constexpr int k_audioSettingsUpdateCounter = 89;
 constexpr int k_chaperoneSettingsUpdateCounter = 101;
 constexpr int k_moveCenterSettingsUpdateCounter = 149;
@@ -121,7 +119,6 @@ public: // I know it's an ugly hack to make them public to enable external
     SettingsTabController m_settingsTabController;
     ReviveTabController m_reviveTabController;
     UtilitiesTabController m_utilitiesTabController;
-    AccessibilityTabController m_accessibilityTabController;
 
 private:
     QPoint getMousePositionForEvent( vr::VREvent_Mouse_t mouse );