[release-branch.go1.11] cmd/go/internal/modcmd: remove non-existent -dir flag

Updates #27243
Fixes #27498

Change-Id: If9230244938dabd03b9afaa6600310df8f97fe92
Reviewed-on: https://go-review.googlesource.com/131775
Reviewed-by: Bryan C. Mills <[email protected]>
(cherry picked from commit 55ef446)
Reviewed-on: https://go-review.googlesource.com/c/146717
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: agnivade

src/cmd/go/alldocs.go

@@ -15,7 +15,7 @@ import (
 )
 
 var cmdDownload = &base.Command{
-	UsageLine: "go mod download [-dir] [-json] [modules]",
+	UsageLine: "go mod download [-json] [modules]",
 	Short:     "download modules to local cache",
 	Long: `
 Download downloads the named modules, which can be module patterns selecting