Remove the builder_name function.

Now that we no longer have multiple milestone-based buckets, it is safe
to refer directly to the bucket when referencing another builder, so the
builder_name function provides no value.

Change-Id: If64a68d4d6e3c153c5d832a656fc2976ef18c7a2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2576945
Auto-Submit: Garrett Beaty <gbeaty@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834320}

infra/config/lib/builders.star

@@ -575,15 +575,8 @@ def builder(
         **kwargs
     )
 
-def builder_name(builder, bucket = args.DEFAULT):
-    bucket = defaults.get_value("bucket", bucket)
-    if bucket == args.COMPUTE:
-        fail("Either a default for bucket must be set or bucket must be passed in")
-    return "{}/{}".format(bucket, builder)
-
 builders = struct(
     builder = builder,
-    builder_name = builder_name,
     cpu = cpu,
     defaults = defaults,
     goma = goma,