[infra/config] make linux_chromium_cfi_rel_ng xenial builderless

We already have 32core machine in luci.chromium.try pool.
https://chromium-swarm.appspot.com/botlist?c=id&c=task&c=os&c=status&d=asc&f=cores%3A32&f=pool%3Aluci.chromium.try&f=builderless%3A1&k=builderless&s=id

Bug: 956776, 954450
Change-Id: I8ee1a5343c01add4717cbc26c60451364aa41d14
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1697123
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Yoshisato Yanagisawa <yyanagisawa@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#676296}

infra/config/cr-buildbucket.cfg

@@ -4119,6 +4119,8 @@ buckets {
     builders {
       mixins: "linux-try"
       mixins: "goma-rbe-prod"
+      mixins: "linux-xenial"
+      mixins: "builderless"
       name: "linux_chromium_cfi_rel_ng"
       dimensions: "cores:32"
     }