Delete all references to GIT_MODE=True from chromium

R=iannucci@chromium.org
BUG=640251

Review-Url: https://codereview.chromium.org/2282933002
Cr-Commit-Position: refs/heads/master@{#414780}

infra/recipes/canary.py

@@ -11,7 +11,7 @@ DEPS = [
 ]
 
 def RunSteps(api):
-  api.gclient.set_config('chromium', GIT_MODE=True)
+  api.gclient.set_config('chromium')
   api.bot_update.ensure_checkout(force=True)
 
 def GenTests(api):