[GCP] Get rid of backup config file and fix identity check for service acocunt

[Michaelvll]

Fixes #3171

We also removed the backup config file, as we got rid of ray autoscaler and will not have the problem where the GCP credential will be overwritten on the remote cluster.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • Reproducible code for in [GCP] GCP should be enabled on the VM with service account assigned #3171
  • sky spot launch -n test-service --cloud gcp --cpus 2 echo hi with spot controller on AWS
  • sky spot launch -n test-service --cloud gcp --cpus 2 echo hi with spot controller on GCP
  • sky launch -c test-fm --cloud gcp ./examples/using_file_mounts.yaml with private buckets, without config set and login to check the GCP credentials used to check whether backup works.
• All smoke tests: pytest tests/test_smoke.py --gcp
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
  • pytest tests/test_smoke.py::test_file_mounts --gcp with private buckets involved
• Backward compatibility tests: bash tests/backward_comaptibility_tests.sh

[cblmemo]

Thanks! LGTM.