systemd doesn't delete stub unit files created for session scopes

Issue

• systemd doesn't delete stub unit files created for session scopes (/run/systemd/system/session-*.scope)

  # ls /run/systemd/system
  ...
  session-10393.scope
  session-10447.scope
  session-10582.scope
  session-11387.scope
  session-11440.scope
  ...
  

• systemctl list-unit-files exits in timeout

  # systemctl list-unit-files
  Failed to list unit files: Connection timed out