Reduce utils.Cfg references (#7650)

* app.UpdateConfig method

* test fix

* another test fix

* the config override option as-was is just error prone, remove it for now

* derp

app/options.go

@@ -4,39 +4,11 @@
 package app
 
 import (
-	"github.com/mattermost/mattermost-server/model"
 	"github.com/mattermost/mattermost-server/store"
 )
 
 type Option func(a *App)
 
-// By default, the app will use a global configuration file. This allows you to override all or part
-// of that configuration.
-//
-// The override parameter must be a *model.Config, func(*model.Config), or func(*model.Config) *model.Config.
-//
-// XXX: Most code will not respect this at the moment. (We need to eliminate utils.Cfg first.)
-func ConfigOverride(override interface{}) Option {
-	return func(a *App) {
-		switch o := override.(type) {
-		case *model.Config:
-			a.configOverride = func(*model.Config) *model.Config {
-				return o
-			}
-		case func(*model.Config):
-			a.configOverride = func(cfg *model.Config) *model.Config {
-				ret := *cfg
-				o(&ret)
-				return &ret
-			}
-		case func(*model.Config) *model.Config:
-			a.configOverride = o
-		default:
-			panic("invalid ConfigOverride")
-		}
-	}
-}
-
 // By default, the app will use the store specified by the configuration. This allows you to
 // construct an app with a different store.
 //