Disable scheduled CI tasks

GitHub disables the workflow completely if you don't have any code changes for 60 days.
Unfortunately they don't only disable the scheduled trigger, but _all_ triggers, so CI wasn't running on PRs either.

Since this repo is mostly dormant, I'm turning off the schedule to prevent this happening in the future.

.github/workflows/ci.yml

@@ -4,8 +4,6 @@ on:
   push:
     branches:
       - master
-  schedule:
-    - cron: "0 0 * * *"
 
 jobs:
   lint: