Make GomaConfig.DEFAULT map to a Goma RBE prod build.

Now that Goma RBE has been vetted switch the requested
GomaConfig.DEFAULT to doing a Goma RBE build rather than a Goma borg
build.

BUG=None
TEST=./run_tests

Change-Id: Icf4c759c798ab13397ee7fdbc349436000959eb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2280920
Tested-by: David Burger <dburger@chromium.org>
Commit-Queue: David Burger <dburger@chromium.org>
Reviewed-by: Alex Klein <saklein@chromium.org>
diff --git a/api/controller/controller_util.py b/api/controller/controller_util.py
index ccfac33..8ea233d 100644
--- a/api/controller/controller_util.py
+++ b/api/controller/controller_util.py
@@ -75,12 +75,11 @@
 
   # Parse the goma config.
   chromeos_goma_dir = goma_message.chromeos_goma_dir or None
-  goma_approach = None
-  if goma_message.goma_approach == common_pb2.GomaConfig.RBE_PROD:
-    goma_approach = goma_util.GomaApproach('?prod', 'goma.chromium.org', True)
-  elif goma_message.goma_approach == common_pb2.GomaConfig.RBE_STAGING:
+  if goma_message.goma_approach == common_pb2.GomaConfig.RBE_STAGING:
     goma_approach = goma_util.GomaApproach('?staging',
                                            'staging-goma.chromium.org', True)
+  else:
+    goma_approach = goma_util.GomaApproach('?prod', 'goma.chromium.org', True)
 
   # Note that we are not specifying the goma log_dir so that goma will create
   # and use a tmp dir for the logs.