[gold] skia_diff_server/main.go: Remove old auth method.
[gold] skia_diff_server/main.go: Remove old auth method.

This CL is based on https://skia-review.googlesource.com/c/buildbot/+/234117.

Change-Id: Ib052094cbe9b4dc0932fbd71b0839e00a2caa2dd
Reviewed-on: https://skia-review.googlesource.com/c/buildbot/+/237996
Commit-Queue: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Reviewed-by: Kevin Lubick <7cdab2cfab351f23814786ba39716e90eed69047@google.com>
Auto-Submit: 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