Remove unused members from Viewport

[HolonProduction]

• gen_mipmaps seems to be a relic from Godot 2: https://docs.godotengine.org/en/2.1/classes/class_viewport.html#class-viewport-set-render-target-gen-mipmaps
• Not sure about last_vp_rect, but blame points to e82dc40

[Ivorforce]

If it compiles, it should be good. It's always a mystery to me though how the last users of some property are removed without the property being removed alongside.

[HolonProduction]

Probably people removing stuff are not always aware that they are removing the last user.

Not sure if there is a way to catch this automatically -Wunused does not seem to catch it.

[Repiteo]

Thanks!