MM-31061: Remove pointers to slice (#16631)

* MM-31061: Remove pointers to slice

This PR removes instances of pointers
to slices in the codebase. There are some other instances in app/import_functions.go
but that's necessary to prevent empty arrays from appearing in the JSON output.

```release-note
NONE
```

https://mattermost.atlassian.net/browse/MM-31061

* fix tests

app/file.go

@@ -218,7 +218,7 @@ func (a *App) ListDirectory(path string) ([]string, *model.AppError) {
 		return nil, model.NewAppError("ListDirectory", "api.file.list_directory.app_error", nil, nErr.Error(), http.StatusInternalServerError)
 	}
 
-	return *paths, nil
+	return paths, nil
 }
 
 func (a *App) RemoveDirectory(path string) *model.AppError {