Remove unused generate-bbs-models script (#4016)

- Protoc was added to the runtime-ci image [1]
- Concourse job bump-bbs-protos was added to CAPI CI [2 + 3]

[1] cloudfoundry/capi-dockerfiles#54
[2] https://concourse.app-runtime-interfaces.ci.cloudfoundry.org/teams/
      capi-team/pipelines/capi/jobs/bump-bbs-protos
[3] https://github.com/cloudfoundry/capi-ci/blob/main/ci/
      update-protobufs/update_bbs_protos.sh

Author: philippthun