Remove scheduled vulnerability scan (#701)

Since the project is now deprecated, vulnerabilities are no longer being
proactively fixed.

Signed-off-by: Mark S. Lewis <[email protected]>

Author: bestbeforetoday

.github/workflows/vulnerability_scan.yml

@@ -1,8 +1,7 @@
 name: "Security vulnerability scan"
 
 on:
-  schedule:
-    - cron: "20 0 * * *"
+  workflow_dispatch:
 
 jobs:
   npm-audit: