Remove the category field from builders.

The category field is unused in LUCI.

Change-Id: Iafb35ecc9b8167b742919bfeb605e5078f647639
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829952Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Garrett Beaty <gbeaty@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700808}

infra/config/cr-buildbucket.cfg

@@ -935,8 +935,6 @@ builder_mixins {
 builder_mixins {
   name: "findit"
   mixins: "goma-many-jobs-for-ci"
-  # This category is meant for Findit to trigger tryjobs on.
-  category: "Chromium Variable"
   recipe {
     name: "findit/chromium/compile"
   }
@@ -3843,7 +3841,6 @@ buckets {
   swarming {
     hostname: "chromium-swarm.appspot.com"
     builder_defaults {
-      category: "Chromium Variable"
       dimensions: "pool:luci.chromium.findit"
       service_account: "findit-builder@chops-service-accounts.iam.gserviceaccount.com"
       execution_timeout_secs: 28800  # 8h
@@ -3903,7 +3900,6 @@ buckets {
     task_template_canary_percentage { value: 5 }
 
     builder_defaults {
-      category: "Chromium CQ"
       dimensions: "cores:8"
       dimensions: "cpu:x86-64"
       service_account: "chromium-try-builder@chops-service-accounts.iam.gserviceaccount.com"