CI: Disable the CI Build on push to master branch

Due to the [recent cost-cutting](apache#14376), we are no longer running PR Merge Jobs in the `nuttx` and `nuttx-apps` repos. For this to happen, I am now running a script on my computer that will cancel any PR Merge Jobs that appear: [kill-push-master.sh](https://github.com/lupyuen/nuttx-release/blob/main/kill-push-master.sh)

This PR disables PR Merge Jobs permanently, so that we no longer need to run the script. This prevents our CI Charges from over-running, in case the script fails to operate properly.

.github/workflows/build.yml

@@ -21,7 +21,6 @@ on:
     paths-ignore:
       - 'Documentation/**'
     branches:
-      - master
       - 'releases/*'
     tags: