Remove last BUCK build file

Buck based build was removed in I6e860446ef, but the new top-level
directory gerrit-test-util with new BUCK build files was added in
I59aa42d89c, after the referenced change was uploaded to review.

Change-Id: I8b3d5bf046be0e6a4bf39ac035c55fbecaa281e0

gerrit-test-util/BUCK

@@ -1,9 +0,0 @@
-java_library(
-  name = 'test_util',
-  srcs = glob(['src/main/java/**/*.java']),
-  visibility = ['PUBLIC'],
-  deps = [
-    '//gerrit-extension-api:api',
-    '//lib:truth',
-  ],
-)