[infra/config] switch to xenial builderless for linux-blink-rel

TBR=jbudorick

Bug: 956776, 954450
Change-Id: I4ef5268679a71be3b1d368fc1eeab30e1aadfa29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1662105Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669591}

infra/config/cr-buildbucket.cfg

@@ -4514,6 +4514,8 @@ buckets {
       name: "linux-blink-rel"
       mixins: "linux"
       mixins: "blink-try"
+      mixins: "linux-xenial"
+      mixins: "builderless"
     }
     builders {
       name: "mac10.10-blink-rel"