Switch mac_chromium_compile_db_ng to builderless

Bug: 999206
Change-Id: I91c01200552c4170ac6d18badd5bdd113396526d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1792461
Commit-Queue: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Auto-Submit: Stephen Martinis <martiniss@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694874}

infra/config/cr-buildbucket.cfg

@@ -4767,6 +4767,7 @@ buckets {
     builders {
       mixins: "mac-try"
       mixins: "goma-j150"
+      mixins: "builderless"
       name: "mac_chromium_compile_dbg_ng"
       dimensions:"os:Mac-10.13"
     }