cmd/go/internal/modget: remove unused base.Errorf argument

cmd/go/internal/modget: remove unused base.Errorf argument

Found by the vet check that runs with 'go test cmd/go/internal/modget'.

For #57001.
For #60463.

Change-Id: I4be94f7156724459a5c47bb9745cbb5651fb972c
Reviewed-on: https://go-review.googlesource.com/c/go/+/498270
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>