Revert "security_tracker: Set service to RemainAfterExit"

This reverts commit 5dd8e051.

Setting RemainAfterExit breaks the timer and prevents it from running
again.

roles/security_tracker/files/security-tracker-update.service

@@ -3,7 +3,6 @@ Description=Security Tracker update service
 
 [Service]
 Type=oneshot
-RemainAfterExit=yes
 User=security
 Group=security
 WorkingDirectory=/srv/http/security-tracker