cmd/go/internal/modload: remove printStackInDie functionality

Previously, when running cmd/go tests, if the module root directory is
requested when modules are explicitly disabled, we printed a stack
trace in addition to the error message that's normally printed. The
stack trace isn't that useful, and it makes the actual error hard to
find.

Change-Id: I8230d668f3f16659f08d0d685124c41b4055c5b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/263659


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