Revert "Switch mac_chromium_compile_db_ng to builderless"

This reverts commit bd916257.

Reason for revert: mac_chromium_compile_dbg_ng is purple:
https://ci.chromium.org/p/chromium/builders/try/mac_chromium_compile_dbg_ng?limit=200

Original change's description:
> 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}

TBR=martiniss@chromium.org,jbudorick@chromium.org

Change-Id: I4a3f86fdb97b98adc7752082e3330f0c2870d6e5
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 999206
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1793263Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694898}

infra/config/cr-buildbucket.cfg

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