Remove obsolete comments about fences am: 2229f5b630

Original change: https://android-review.googlesource.com/c/platform/art/+/2124267

Change-Id: I9876c0a44494a055b86be0b7165ebfe104b3ee27
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>