Remove npm caching from the example GHA workflow.

This is difficult to set up because a `package-lock.json` file is required in order to create a cache key. In this context, the lock file should be the one from WordPress Core. But since WordPress is not checked out until the `prepare.php` step, caching cannot be configured before hand.

Author: desrosj

.github/workflows/tests.yml

@@ -75,7 +75,6 @@ jobs:
         uses: actions/setup-node@25316bbc1f10ac9d8798711f44914b1cf3c4e954 # v2.4.0
         with:
           node-version: 14
-          cache: npm
 
       - name: Prepare environment
         run: php prepare.php