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
morgankrey
af050fc565