cmd/go/internal/modload: don't set GIT_SSH_COMMAND

cmd/go/internal/modload: don't set GIT_SSH_COMMAND

Allows git core.sshcommand to take effect when set by the user.
This was originally added to work around an issue in
OpenSSH < 7.3 (2016), see https://go.dev/issue/13453 .
A fixed version of OpenSSH should be widely available enough
that it is no longer necessary

Fixes #71482

Change-Id: I6f44cc354e8a4063e226cac78ec27117fcc40e93
Reviewed-on: https://go-review.googlesource.com/c/go/+/647995
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Auto-Submit: Daniel Martí <mvdan@mvdan.cc>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>