Fact is, if the game is looking for those files then there is some reference/use to them in your game.
Whether you remember using them or not doesn't change the fact that the references are in your game.
It might be that they were automatically included when you loaded a sample map or copied an event from another game or as defaults from a plugin or something like this, but as of now those references are in your game, and you have only two options now: either copy those files from the base resources into your game, or find and change the parts where those battlebacks are required.
If you want to remove these references, first area to check would be all map properties for the default battlebacks set there. If its somewhere else it will be more difficult to find.
I've moved this thread to MV Support. Please be sure to post your threads in the correct forum next time. Thank you.
Tech support is for getting the editors to run.