cmd/go/internal/modcmd: remove dead function addModFlag
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 <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

cloudflare/go,CAFxX/go,golang/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,cloudflare/go,golang/go,nawawi/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,akutz/go,golang/go,golang/go,golang/go,cloudflare/go,akutz/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,akutz/go,golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,CAFxX/go