Fix git auth conflict in background_agent_mvp workflow (#49537)

Fix git auth conflict in background_agent_mvp workflow

The workflow was manually configuring git authentication with:
```bash
git config --local http.https://github.com/.extraheader "AUTHORIZATION: bearer ..."
```

This conflicted with the authentication already set up by
`actions/checkout@v4`, which uses `AUTHORIZATION: basic ...`.

The conflict caused all crash pipeline runs to fail with:
```
fatal: could not read Username for 'https://github.com'
```

This morning's run (22147984206) failed for all 15 crash candidates
(ZED-4VS, ZED-202, etc.) with this error.

Remove the redundant git config since actions/checkout already handles
authentication properly.

Release Notes:

- N/A

.github/workflows/background_agent_mvp.yml

@@ -148,8 +148,6 @@ jobs:
               continue
             fi
 
-            git config --local http.https://github.com/.extraheader "AUTHORIZATION: bearer ${GH_TOKEN}"
-
             if ! git fetch origin main; then
               echo "WARNING: Failed to fetch origin/main for $CRASH_ID — skipping"
               continue