hotfix(backend): Remove GH header token middleware

[amanape]

End-user friendly description of the problem this fixes or functionality that this introduces

• The middleware messed with the saas /api/github/callback endpoint since it also starts with /api/github. This was only meant to apply to requests under github.py

• Include this change in the Release Notes. If checked, you must provide an end-user friendly description for your change below

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Give a summary of what the PR does, explaining any non-trivial design decisions

• Use route specific dependencies instead of app-wide middleware (see https://fastapi.tiangolo.com/tutorial/dependencies/#create-a-dependency-or-dependable)

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Link of any specific issues this addresses

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To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:2f4bb12-nikolaik   --name openhands-app-2f4bb12   docker.all-hands.dev/all-hands-ai/openhands:2f4bb12

[neubig]

Looks fine to me. In either this or a future PR could we add a test to make sure that this doesn't regress?