Update Github migration test (#8896)

Earlier today #716 was reopened which updated the modification time for
an old milestone (1.6.0) that we use in testing with the assumption that
it is old and won't change. This breaks all builds now, so remove this
test since we have others that test the same code and this milestone
will likely be updated again as that issue changes etc...

modules/migrations/github_test.go

@@ -118,12 +118,6 @@ func TestGitHubDownloadRepo(t *testing.T) {
  "2018-09-05 16:34:22 +0000 UTC",
  "2018-08-11 08:45:01 +0000 UTC",
  "closed", milestone)
- case "1.6.0":
- assertMilestoneEqual(t, "1.6.0", "2018-09-25 07:00:00 +0000 UTC",
- "2018-05-11 05:37:01 +0000 UTC",
- "2019-01-27 19:21:22 +0000 UTC",
- "2018-11-23 13:23:16 +0000 UTC",
- "closed", milestone)
  case "1.7.0":
  assertMilestoneEqual(t, "1.7.0", "2018-12-25 08:00:00 +0000 UTC",
  "2018-08-28 14:20:14 +0000 UTC",