refactor: move private buildpacks function to functions.go

plugins/buildpacks/subcommands.go

@@ -167,13 +167,3 @@ func CommandSet(args []string, index int) (err error) {
 
 	return
 }
-
-func getAppName(args []string) (appName string, err error) {
-	if len(args) >= 1 {
-		appName = args[0]
-	} else {
-		err = errors.New("Please specify an app to run the command on")
-	}
-
-	return
-}