Given the gitaly-* now proxy the data from the client to the Gitaly
server, the environment variables aren't used. Therefor we don't have to
set them either. Only exception to the rule, is the GITALY_TOKEN.

These changes also remove the `GIT_TRACE` options, introduced by
192e2bd3.

Part of: https://gitlab.com/gitlab-org/gitaly/issues/1300