fix: remove redefinition of github token

Fixes a regression introduced in flarum#4078 in relation to composer auth.

It appears as if explicitly defining it in the job interferes with how actions such as shivammathur/setup-php@v2 authenticate with composer.

Author: DavideIadeluca

.github/workflows/REUSABLE_frontend.yml

@@ -124,7 +124,6 @@ jobs:
     name: Checks & Build
     runs-on: ${{ inputs.runner_type }}
     env:
-      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
       GIT_ACTOR_TOKEN: ${{ secrets.git_actor_token }}
 
     if: >-