cmd/go/internal/modfetch: drop gopkg.in/russross/blackfriday.v2 from TestCodeRepoVersions

Follow-up for CL 265819.

Given the -pre tag added recently, a new stable version is likely
tagged soon. This would break TestCodeRepoVersions on the longtest
builders again. Since the other test cases in codeRepoVersionsTests
already provide enough coverage, drop gopkg.in/russross/blackfriday.v2
to avoid breaking TestCodeRepoVersions once the release happens.

Updates #28856

Change-Id: If86a637b5e47f59faf9048fc1cbbae6e8f1dcc53
Reviewed-on: https://go-review.googlesource.com/c/go/+/265917
Trust: Tobias Klauser <[email protected]>
Run-TryBot: Tobias Klauser <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: tklauser

src/cmd/go/internal/modfetch/coderepo_test.go

@@ -645,11 +645,6 @@ var codeRepoVersionsTests = []struct {
 		path:     "swtch.com/testmod",
 		versions: []string{"v1.0.0", "v1.1.1"},
 	},
-	{
-		vcs:      "git",
-		path:     "gopkg.in/russross/blackfriday.v2",
-		versions: []string{"v2.0.0", "v2.0.1", "v2.1.0-pre.1"},
-	},
 	{
 		vcs:      "git",
 		path:     "gopkg.in/natefinch/lumberjack.v2",