Remove U2F support (#20141)

- Completely remove U2F support from 1.18.0, 1.17.0 will be the last
release that U2F is somewhat supported. Users who used U2F would already
be warned about using U2F for a while now and should hopefully already
be migrated. But starting 1.18 definitely remove it.

routers/web/user/setting/security/security.go

@@ -26,7 +26,6 @@ const (
 func Security(ctx *context.Context) {
 	ctx.Data["Title"] = ctx.Tr("settings")
 	ctx.Data["PageIsSettingsSecurity"] = true
-	ctx.Data["RequireU2F"] = true
 
 	if ctx.FormString("openid.return_to") != "" {
 		settingsOpenIDVerify(ctx)