Remove intermediate storage of environment Canada data

**Describe the task**
Get rid of intermediate tables in database (such as prediction_model_grid_subsets and model_run_grid_subset_predictions); store only interpolated values.

**Acceptance Criteria**
- [ ] Intermediate data no longer stored in database.
- [ ] prediction_model_grid_subsets removed.
- [ ] model_run_grid_subset_predictions removed.

**Additional context**
- We're storing the four point around a weather station, in addition to the interpolated values for the weather station. It is unlikely that the four points will be used in the future, and takes up a lot of space.