Remove gRPC

[TheMrSheldon]

The gRPC endpoints are completely legacy and only used by the runners (in the pipelines folder). They amount for a large part of legacy code that should also be removed as part of this ticket. AFAIK at least the following files should be modified or removed:

• application/src/run_grpc_server.py
• gRPC endpoints in application/src/tira/urls.py and large parts of vm_api since they are only used by the gRPC endpoints
• the runners in pipelines/src/bash -- the python3 snippets may possibly be removed with wget or curl to our REST-API
• The entire protocol folder since it only contains the gRPC definitions and its generated output
• Remove reroute_host in application/src/tira/util.py
• Remove grpc_host setting from the django configs

The gRPC endpoints that can not be deleted without substitution should be replaced using REST endpoints. If none of the REST endpoints in our current specification (documentation/development/specification/rest-api.yml) fit the need, please add these endpoints to the specification in compliance with our REST-API guidelines (https://tira-io.github.io/tira/nightly/development/specification/restapi.html).