debt: remove redundant soil data mutations and restructure `test_soil_data.py`

[garobrik]

Once all of our clients are using the pushSoilData mutation, there will be 4 redundant mutations:

• updateSoilData
• updateDepthDependentSoilData
• updateSoilDataDepthInterval
• deleteSoilDataDepthInterval

To keep our code surface smaller, we should remove these mutations.

Once we do that, we'll have to change most of the tests in test_soil_data.py which are currently testing these endpoints. Many of those tests are testing constraints and permissions conditions that should still be tested, but it might make more sense to test those at a different layer than graphql queries. So most of the work for this issue will be figuring out the right way to refactor that test suite.