[release-branch.go1.13] cmd/go/internal/get: remove '--' separator from 'git ls-remote' command

'git ls-remote' started recognizing the '--' separator at some point
after 2.7.4, but git defaults to version 2.7.4 on Ubuntu 16.04 LTS,
which remains supported by Ubuntu until April 2021.

We added '--' tokens to most VCS commands as a defensive measure in
CL 181237, but it isn't strictly necessary here because the 'scheme'
argument to our template is chosen from a predefined list: we can
safely drop it to retain compatibility.

Cherry-picked from CL 191978.

Updates #33836
Fixes #33880

Change-Id: Ibb53366b95f8029b587e0b7646a439330d759ac7
Reviewed-on: https://go-review.googlesource.com/c/go/+/191973


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>