Remove vendor directory

Overview

GitLab Runner is now using Go 1.13 with modules enabled. We also have the vendor directory committed to source code, which is the "old" way of handing dependencies during the build time. We already have some jobs doing this such as rpm package signing by accident because they ignore the vendor directory. We should remove the vendor directory completely and rely on go modules to solve everything during the build time. We already have some projects doing this such as GitLab Pages, GitLab Workhosre.

We don't have to worry about a package being deleted off GitHub since we are using Google's GOPROXY by default.

Also AFAIK the vendor directory is only checked when the project is inside of a GOPATH that is why jobs like rpm package signing, we also would need to update our build commands to explicitly use the vendor directory https://www.reddit.com/r/golang/comments/9ai79z/correct_usage_of_go_modules_vendor_still_connects/.

Benefits

• Follow Go best practices
• Less cruft in source code
• We wouldn't have to keep ignoring vendor directory for scripts such as go list