Ensure babel.config.js is not used without useBabelConfig option

We have been disabling loading of `.babelrc` files for quite some time,
but missed the addition of `babel.config.js` to `@babel/core`'s
configuration story.

Now that we are attempting to roll out a version of ember-cli-babel that
primarily leverages `babel.config.js` this mistake is coming back to
haunt us. Without this explicit disabling of the `options.configFile`
option ember-cli-babel versions **prior to** the ones that are adding
support for `babel.config.js` (and wihtout opting in) are parsing
`babel.config.js` files and ultimately breaking their builds.

Note: this should not be a breaking change as we did not really support
this before (and build errors would have always occurred), but without
this change it is very difficult to adopt [email protected]'s new
`useBabelConfig` option.

index.js

@@ -330,6 +330,7 @@ module.exports = {
 
     options.highlightCode = this._shouldHighlightCode();
     options.babelrc = false;
+    options.configFile = false;
 
     return options;
   },