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.

go.mod

@@ -1,6 +1,6 @@
 module github.com/mattermost/mattermost-server/v5
 
-go 1.12
+go 1.14
 
 require (
 	github.com/Masterminds/squirrel v1.1.0