[BrianG]

The external BEC will backfeed into the internal BEC and may cause problems. IMO, it's best to avoid potential issues and remove the wire.

And, you don't have to cut it or anything, just gently pry up the little plastic clip in the connector, slide the red wire out, fold it back, and heatshrink the red wire to the other two. That way, you can re-enable it someday if you want. Or, get a short servo extension cable and remove the red wire from both ends. This way, no mods are needed to the ESC cable. Either way works the same.