[infra] Remove regen_test_json call from unit test.

The regen_test_json function is intended to simplify the process of
updating the test json files when making changes, but not intended to be
left in when submitted. A use of it was accidentally committed in
crrev.com/c/4632065.

Bug: 1456553
Change-Id: I950fbd480baa600bdb00c2e7ed8b20bca195eb3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4651991
Reviewed-by: Struan Shrimpton <sshrimp@google.com>
Commit-Queue: Struan Shrimpton <sshrimp@google.com>
Auto-Submit: Garrett Beaty <gbeaty@google.com>
Cr-Commit-Position: refs/heads/main@{#1163047}

testing/buildbot/generate_buildbot_json_unittest.py

@@ -2985,7 +2985,6 @@ def test_args_field_merging(self):
                     FOO_TEST_SUITE_WITH_ARGS,
                     LUCI_MILO_CFG,
                     mixins=MIXIN_ARGS)
-    self.regen_test_json(fbb)
     fbb.check_output_file_consistency(verbose=True)
     self.assertFalse(fbb.printed_lines)