[gold] Minor diffcalculator fixes
[gold] Minor diffcalculator fixes

We need less tracing, since performance is ok for now.

We don't need to make a go routine to make a go routine.

Change-Id: I1fef3886821e98ef28ada3210a2f4d8c950f9ced
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/377944
Reviewed-by: Leandro Lovisolo <f558143c2568be8d55575d13f30a1fc5cb8009d5@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot