[cr-buildbucket] Clear cores for linux_chromium_cfi_rel_ng to test beefier machines.

BUG=731417

Change-Id: Ic2e8071ce052e2d8ef5b6f0c508f6f62da7624fc
Reviewed-on: https://chromium-review.googlesource.com/1089913Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Commit-Queue: Jao-ke Chin-Lee <jchinlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565065}

infra/config/global/cr-buildbucket.cfg

@@ -2390,7 +2390,11 @@ buckets {
     builders { mixins: "linux-try" name: "linux_chromium_asan_variable" }
     builders { mixins: "linux-try" name: "linux_chromium_browser_side_navigation_rel" }
     builders { mixins: "linux-try" name: "linux_chromium_cast_variable" }
-    builders { mixins: "linux-try" name: "linux_chromium_cfi_rel_ng" }
+    builders {
+      mixins: "linux-try"
+      name: "linux_chromium_cfi_rel_ng"
+      dimensions: "cores:" # clear cores dimension while testing beefier bots
+    }
     builders { mixins: "linux-try" name: "linux_chromium_chromeos_asan_rel_ng" }
     builders { mixins: "linux-try" name: "linux_chromium_chromeos_asan_variable" }
     builders { mixins: "linux-try" name: "linux_chromium_chromeos_compile_dbg_ng" }