Adjust "lint:ignore" comments for golangci-lint
In a lot of places we're using "lint:ignore" comments to ignore warnings the linter would normally complain about. Since we've migrated to golangci-lint in 713d1e48 (Use golangci-lint for static code analysis, 2020-02-06), these don't do anything anymore since golangci-lint instead uses a different format: instead of "lint:ignore", we need to add "nolint" comments.
We should thus review and adjust all current places where we use the old syntax and either remove it, if it doesn't apply anymore, or update it to use the new syntax.