cmd/go/internal/modcmd: remove non-existent -dir flag

Fixes #27243

Change-Id: If9230244938dabd03b9afaa6600310df8f97fe92
Reviewed-on: https://go-review.googlesource.com/131775Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/alldocs.go

@@ -889,7 +889,7 @@
 //
 // Usage:
 //
-// 	go mod download [-dir] [-json] [modules]
+// 	go mod download [-json] [modules]
 //
 // Download downloads the named modules, which can be module patterns selecting
 // dependencies of the main module or module queries of the form path@version.

src/cmd/go/internal/modcmd/download.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