[infra] fix mixins for linux-chromeos-rel

I intended to make CQ builderless instead of CI in
https://chromium-review.googlesource.com/c/chromium/src/+/1645452

Bug: 954450
Change-Id: I27d546b8ce8e619915133c66e08203f80ee167d4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1647635
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666587}

infra/config/cr-buildbucket.cfg

@@ -1338,7 +1338,6 @@ buckets {
       name: "linux-chromeos-rel"
       mixins: "chromeos-ci"
       mixins: "linux-xenial"
-      mixins: "builderless"
     }
 
     builders {
@@ -3639,6 +3638,7 @@ buckets {
       mixins: "chromeos-try"
       mixins: "linux-xenial"
       mixins: "goma-j150"
+      mixins: "builderless"
       name: "linux-chromeos-rel"
     }