UI: Remove unused RemoveSelectedSceneItem slot

Appears to be a remnant left by the undo/redo system changes, not used
at all by any parts of the UI. This slot does not have undo/redo
system functionality, so it is not useful in its current form.
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f