cmd/go/internal/modload: remove the needSum argument from the fetch function

cmd/go/internal/modload: remove the needSum argument from the fetch function

With moduleHasRootPackage eliminated in the previous CL, needSum is
now invariantly true at all call sites.

Change-Id: I00e44117e545ea0d3de82604dfa018b013ab8f0c
Reviewed-on: https://go-review.googlesource.com/c/go/+/440296
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>