remove DMJsonWriter::AddTestFailure

As far as I can tell, Gold doesn't do anything with this information.
The bots go red if any unit test fails... no need to log it long-term.

Change-Id: Iae9ddd61c0a7d37437bb93441e83948c2e518b42
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/206940
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed