How do I delete/remove a story map I no longer need
If you are a workspace administrator, you can delete story maps from the workspace dashboard / Manage / Manage Storymaps menu. There you'll see a "More" button and under that you'll find Delete.
If you are a Story map admin, you can delete a board from the board's ... menu / Board settings / General tab.
Please note, that deleting a map is unrecoverable.