Remove Bot-Commit label permissions from refs/changes/*.

Label permissions are supposed to be applied on the target ref, not refs/changes/*.

Change-Id: I86f7488d147f2ef00f9224a31975a8cf7048d4c8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/+/6515822
Reviewed-by: Andrew Mitchell <mitchella@google.com>