GitHubActions: fix invalid format of require-pr-porting-labels.yaml

The yaml file has an indent issue from line 15.

And the branches filter should be under pull_request_target but
not the pull_request trigger.

Also actions/checkout@v2 does not need the token parameter.

Fixes: #2798

Signed-off-by: bin <bin@hyper.sh>

.github/workflows/require-pr-porting-labels.yaml

@@ -12,7 +12,6 @@ on:
       - reopened
       - labeled
       - unlabeled
-   pull_request:
     branches:
       - main
 
@@ -32,8 +31,6 @@ jobs:
 
       - name: Checkout code to allow hub to communicate with the project
         uses: actions/checkout@v2
-        with:
-          token: ${{ secrets.KATA_GITHUB_ACTIONS_TOKEN }}
 
       - name: Install porting checker script
         run: |