nova db-migrate: remove --local_cell parameter
nova db-migrate: remove --local_cell parameter

The --local_cell parameter was initially introduced [1] to help
upgrading nova to cellv2.
That's been done since a while ago, so we need to remove this
parameter again so that db migrations will run against all the
cells databases and cell0.

[1] https://github.com/sapcc/helm-charts/commit/34943d0ecb260d76dd50c451a27d3d6139eebb26

sapcc/helm-charts,sapcc/helm-charts,sapcc/helm-charts,sapcc/helm-charts