[release/0.9] Remove vendor dir in /test (#1417)

[helsaawy]

Given this slows down development both for us and external contributors as for most changes one would need to run go mod vendor in /test to bring in the latest local hcsshim changes, I think it's time we removed this.

Pros:

1. Easier for automated tooling like dependabot, and more recently a Microsoft security bot, to make PRs that can just be checked in. All of these automated PRs tend to fail as the bot doesn't know it would need to run go mod vendor in /test as well for our repo. The approach today to check these in is typically someone manually checks out the branch dependabot (or whatever other bot) made, vendor to test, and then push a new commit to those automated PRs and then we can check them in.

2. Speeds up development flow as we don't need to go mod vendor in test before pushing almost every change.

3. Speeds up external contributions as well as there's no extra step to follow to make a change to most things in /internal anymore. We state that this needs to be done in our README, but it's probably a testament to how odd our setup is that it's missed here and there.

Cons:

1. We lose the main selling point of vendoring for our test dependencies which is that if one of our dependencies is no longer accessible (deleted, renamed, whatever else) we don't have a local copy included in our repo. This will increase our dependence on the Go modules proxy server which seems like a fair tradeoff, and I think we're fine with this for test dependencies at least.

I've removed the references to this extra step in the README as well as got rid of the CI step verifying that the vendor dir was up to date. I don't think we needed the mod=vendor env var either, as since go 1.14 if there's a top level vendor folder I believe the flag is transparently set for commands that accept it.

(cherry picked from commit 4e60239)