Remove os:10.12 from mac_chromium_dbg_ng's requested dimensions.

Its machines were upgraded to 10.13 a while ago, so it's been unable to
target any of them since.

R=jbudorick, martiniss

Bug: 945337
Change-Id: I9e85241384f9c2b0121a4570364888ba776c0808
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559886Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649188}

infra/config/cr-buildbucket.cfg

@@ -3672,7 +3672,7 @@ buckets {
       dimensions:"os:Mac-10.13"
     }
     builders { mixins: "mac-try" name: "mac_chromium_compile_rel_ng" }
-    builders { mixins: "mac-try" name: "mac_chromium_dbg_ng" dimensions:"os:Mac-10.12"}
+    builders { mixins: "mac-try" name: "mac_chromium_dbg_ng" }
     builders {
       mixins: "mac-try"
       mixins: "goma-j150"