clean up legacy "global_extent" logic

[soxofaan]

This is a "future work" ticket to clean up legacy "global_extent" logic (in _extract_load_parameters)
once the new approach (post-dry-run) has fully replaced that.

refs:

• Revise global extent computation & alignment #406
• load_stac: improved alignment to native grid openeo-geopyspark-driver#1299
• Use post_dry_run to calculate global extent openeo-geopyspark-driver#1437
• Eliminate load params dependency from construct_item_collection openeo-geopyspark-driver#1435
• Initial support for load_stac in _extract_spatial_extent_from_constraint openeo-geopyspark-driver#1446