Remove `go_get_handle_401_error` feature flag

What does this MR do and why?

Contributes to #496539 (closed)

Removes the feature flag.

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 !161162 (merged).

Solution

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