should use goma-rbe-prod-ats instead of specify the one by one.

Since the value is overwritten by latter mixin, we should use
goma-rbe-prod-ats instead of specifying goma-rbe-prod and goma-ats.

Bug: 981433
Change-Id: I4f4ffd74f01a5c028e68c8a5c6e91506b6077917
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1763659Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689366}

infra/config/cr-buildbucket.cfg

@@ -3483,8 +3483,7 @@ buckets {
       name: "chromeos-amd64-generic-rel-goma-rbe-canary"
       mixins: "builderless"
       mixins: "fyi-ci"
-      mixins: "goma-ats"
-      mixins: "goma-rbe-prod"
+      mixins: "goma-rbe-prod-ats"
       mixins: "linux-xenial"
     }
     builders {
@@ -3638,8 +3637,7 @@ buckets {
       name: "chromeos-amd64-generic-rel-goma-rbe-latest"
       mixins: "builderless"
       mixins: "fyi-ci"
-      mixins: "goma-ats"
-      mixins: "goma-rbe-prod"
+      mixins: "goma-rbe-prod-ats"
       mixins: "linux-xenial"
     }
     builders {