User symlink files in /run/ are not deleted when server is stopped, causing 500 internal server error

### Bug:
If a user stops their JupyterHub server through their control panel and then attempts to start it again, they will receive an error when systemdspawner spawns the user:

"500 : Internal Server Error Error in Authenticator.pre_spawn_start: FileExistsError [Errno 17] File exists: '/run/jupyter-username-singleuser'

The user cannot connect until I manually delete the user's symlink file in /run/.

Perhaps this is a bug where the user's symlink isn't automatically deleted from /run/ when they stop their server. When the spawner tries to start their server again, it erroneously thinks the server is already running and throws the 500 error.

### Additional information:

I'm using a GitLab OAuthenticator with systemdspawner's dynamic_spawning set to true, if that helps.

Ubuntu 20.04
JupyterHub 1.2.2
Latest version of systemdspawner as of Dec. 15, 2020