Investigate removing inner sub_plans

On grids and rotations we have inner sub_plans of `DO_FGS` and `GRIDSCAN_MAIN_PLAN`. I believe that these are only needed so that we have a second thing to trigger zocalo on after the ispyb depositions have been made. Following https://github.com/DiamondLightSource/hyperion/pull/1198 the Zocalo plan can be sure that the depositions are made when it gets it's first start document as it has the ids in the metadata. This may mean we can remove the inner sub_plans and make the plans a lot tidier.

### Notes
* We would still need something to trigger the end off, this could be an event like the detector disarming (though does this work for multiple rotations in one arm)?
* We would need to think about how we calculate processing time, this could be based on events from `ZocaloResults`?

## Acceptance Criteria
* We investigate why we still need inner sub-plans
* If they are necessary we document them better
* If not, we remove them