Revert "my_activity: Add review.coreboot.org as a source"

This reverts https://chromium-review.googlesource.com/1173069

It breaks my_activity.py on Windows.

R=iannucci@chromium.org, rrangel@chromium.org

Bug: 888641
Change-Id: I353489e52155d40e48e63819dc3016cf7dd59277
Reviewed-on: https://chromium-review.googlesource.com/1241133Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>

my_activity.py

@@ -108,9 +108,6 @@ gerrit_instances = [
   {
     'url': 'android-review.googlesource.com',
   },
-  {
-    'url': 'review.coreboot.org',
-  },
   {
     'url': 'pdfium-review.googlesource.com',
   },