cmd/go/internal/modload: remove go116EnableNarrowAll constant

This constant existed in case there was a serious problem with the
change to the "all" pattern in Go 1.16 (CL 240623), so that we could
roll back the change in behavior by just flipping the constant without
introducing merge conflicts elsewhere.

Go 1.16 has been out for a while and the new "all" behavior seems fine,
so we can jettison this feature flag.

For #36460

Change-Id: Ic2730edcee81514d56c7086e11542468eb63c84a
Reviewed-on: https://go-review.googlesource.com/c/go/+/303431


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>