use big back button instead of small one

[thunderrabbit]

There's a small back button that needs to be removed altogether

It should be replaced with the big button that appears after the game is won/ lost

[thunderrabbit]

https://github.com/thunderrabbit/conswi/wiki/scene-level_select_screen

[thunderrabbit]

The small button is 'res://images/hud_buttons/back@3x.png' and appears in HUD.tscn

The large button is too big to fit during game play as it would cover up the required quantity of tiles:

In these photos, there is a red dot on the small button just to identify it.