Don't execute CodeQL via cron

this scanner is running on pull requests and commits to the main branch
and we don't really need to execute it via cron. When the scheduler is
configured GitHub disables this workflow when there are 60 days of
inactivity on the repository.

Author: atodorov

.github/workflows/codeql.yml

@@ -5,8 +5,6 @@ on:
     branches: [ "master" ]
   pull_request:
     branches: [ "master" ]
-  schedule:
-    - cron: "42 21 * * 2"
 
 jobs:
   analyze: