Game Buttons should remove accessible buttons on destroy

[FostUK]

• In accessibilify.js we hook into the destroy method of an event. The accessible button should also destroy itself here and clean up any arrays it is part of.
• sys.accessibleButtons is for gameButttons and should be renamed gameButtons
• removeButton should be removed as it is not used internally and agencies can just destroy their accessibilified game objects safely with the above changes.

[binhbui]

removeButton is used by storytime right?

[AdamXA]

I guess the idea will be that future versions of Genie will make it so removeButton is not required anymore, just destroy the object and the accessibilify stuff is cleaned up automatically.

Storytime can use the old version of Genie and continue with removeButton, or update to a new version of Genie with this change in and stop calling removeButton.