db/models:

Removed some more unused code.
tests/calcs/hazard/classical/core_test:

Removed an obsolete test.
tests/db/models_manager_test:

Removed obsolete tests for the HazardCurveDataManager.
tests/calcs/hazard/classical/post_processing_test:

Decimal inputs for `test_weighted_quantile_with_decimal_weights` are now
constructed from strings, to more closely match the calculator code.

Also added another comment to better explain what the test case is doing.