Remove signal_id from deployments API and model

The same information is now available from the deployment's
derived config deploy_signal_id input value.

Other deployment signal transports might not have a single ID
to put in this field so it is better at this stage to remove
it from the deployments model and do anything that is required
using derived config inputs.

partial blueprint hot-software-config

Change-Id: I90c85d0ceb9fd67eed640c84348ef4175a6194b6
Merge "Remove signal_id from deployments API and model"