Remove drift compatibility for measurements

[rolandgeider]

Remove the compat layer added in df2791a.

• lib/database/powersync/tables/measurements.dart: the .nullable() on metric_type, order, is_official and source, plus the comment above them, and the constructor: 'fromDb' on both @UseRowClass annotations
• lib/features/measurements/models/measurement_category.dart and measurement_entry.dart: the fromDb constructors
• lib/features/measurements/providers/measurement_repository.dart: the isNotValue(i) in reorderCategories, which only needs to be NULL-safe because of the legacy rows

Needs #1319 merged