Shaders: Remove unused water surface shader

Shaders: Remove unused water surface shader

Also remove hardcoded MTGame node.

The 'water surface shader' was duplicated shader code in preparation for
intended new water surface shaders. For development purposes the MTGame node
'default:water_source' had it's top tile assigned to 'water surface shader'.
Due to shader duplication this commit does not cause any change to shader
behaviour.