workflows: Remove the possibility to manually trigger the nightly CI

As a new workflow was added for the cases where developers want to test
their changes in the workflow itself, let's make sure we stop allowing
manual triggers on this workflow, which can lead to a polluted /
misleading weather of the CI.

Signed-off-by: Fabiano Fidêncio <fabiano@fidencio.org>

.github/workflows/ci-nightly.yaml

@@ -2,7 +2,6 @@ name: Kata Containers Nightly CI
 on:
   schedule:
     - cron: '0 0 * * *'
-  workflow_dispatch:
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}