[Monitoring] remove reference to .monitoring-*mb* indices when monitoring with metricbeat in 8.0

[igoristic]

As part of #73864 effort we should decide if we want to also support the "old" metricbeat monitoring indices (.monitoring-*mb*), since we will now be querying from metricbeat-* indices. We need to also consider how it affects what we currently support with 7.x (and will support in 8.x) based on our support matrix

If we will not be supporting .monitoring-*mb* indices along side metricbeat-* indices, then I would consider that a "breaking" change and we need to treat it as such (and decide whether there's a migration process). We would also need to remove it from the kibana code

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)