Remove unnecessary blank imports (#23283)

They aren't necessary after the monorepo

```release-note
NONE
```

server/channels/imports/boards_imports.go

@@ -1,9 +0,0 @@
-// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
-// See LICENSE.txt for license information.
-
-package imports
-
-import (
-	// Needed to ensure the init() method in the FocalBoard product is run.
-	_ "github.com/mattermost/mattermost-server/server/v8/boards/product"
-)

server/channels/imports/playbooks_imports.go

@@ -1,9 +0,0 @@
-// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
-// See LICENSE.txt for license information.
-
-package imports
-
-import (
-	// Needed to ensure the init() method in the Playbooks product is run.
-	_ "github.com/mattermost/mattermost-server/server/v8/playbooks/product"
-)