Remove model_workflow_branch and model_workflow_revision from request.data

See if these values are still needed. If they are not, remove them from the request and any references to them.

https://github.com/MetOffice/CDDS/blob/6332a0e8415bd949ad7160f043da5e7fd65f066e/cdds/cdds/common/request/data_section.py#L48-L49

Start with cdds/cdds/common/request/data_section.py at lines 48-49 and inspect how model_workflow_branch and model_workflow_revision are populated and consumed. Search the repository for both names to determine whether they are still needed and identify every reference. Done means removing the request fields and any references if they are unused, with relevant checks passing.

Written by the indexing model from the issue text.