MM-23817: Use auto-vendoring (#14231)

With Go 1.14 (https://tip.golang.org/doc/go1.14#go-command),
it now automatically checks for the existence of a vendor directory.

So there is no need to explicitly set the `-mod=vendor` flag.

config/default.go

@@ -1,4 +1,4 @@
-//go:generate go run -mod=vendor config_generator/main.go
+//go:generate go run config_generator/main.go
 
 // Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
 // See LICENSE.txt for license information.