cmd/go/internal/modcmd: remove dead function addModFlag

This function is never called and should have been removed
earlier. work.AddModCommonFlags defines the -modfile flag instead.

Fixes #37189

Change-Id: I73ad2a727013a849cba44bf70de04160f37c97dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/219197
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Bryan C. Mills <[email protected]>

Author: Jay Conrod

src/cmd/go/internal/modcmd/mod.go

@@ -7,7 +7,6 @@ package modcmd
 
 import (
 	"cmd/go/internal/base"
-	"cmd/go/internal/cfg"
 )
 
 var CmdMod = &base.Command{
@@ -32,7 +31,3 @@ See 'go help modules' for an overview of module functionality.
 		cmdWhy,
 	},
 }
-
-func addModFlags(cmd *base.Command) {
-	cmd.Flag.StringVar(&cfg.ModFile, "modfile", "", "")
-}