cmd/go/internal/web: remove a redundant return value

cmd/go/internal/web: remove a redundant return value

The URL return value from the fetch helper in web.get was always
either the passed in URL (on success) or nil (on failure).
Remove it to reduce code complexity.

For #61877.

Change-Id: I0ce4602b921d1c996aa988e7d3f83996511ccd72
Reviewed-on: https://go-review.googlesource.com/c/go/+/518016
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>