Remove `go_get_handle_401_error` feature flag

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/496539

**Original problem**

Self-managed instances that restricted password authentication for Git
over HTTP(S) started to receive 401 error code for `go-get=1` requests
from go toolchain.

The reason is a missing return for the case when request doesn't have
basic credentials.

It was introduced in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/161162.

**Solution**

Restore check for missing basic credentials and add a test case.

Changelog: fixed