I realized that on my backup NAS, I didn't need the Symlinks plugin so I uninstalled it (after removing the one existing symlink I had).
The 'Symlinks' entry is still showing up under Services in the GUI, and selecting it produces an unsurprising RPC service 'Symlinks' not found error.
So the actual plugin is gone, but the GUI tab won't go away. I re-installed and uninstalled again, but that didn't help. How to fix?