[release-branch.go1.14] cmd/go/internal/modfetch: drop...

[release-branch.go1.14] 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 <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
(cherry picked from commit 421d4e72)
Reviewed-on: https://go-review.googlesource.com/c/go/+/266178


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
(cherry picked from commit 8687f6d9)
Reviewed-on: https://go-review.googlesource.com/c/go/+/266302


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>