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Geometry Nodes: decouple Delete Geometry node from Mask modifier

This dependency was a bit ugly and the functions from the mask modifier
did a few things that we don't need in the Delete Geometry node:
* The mask modifier uses `CustomData_copy_data` which the node doesn't need.
* The mask modifier checks for `-2` in some values, but this special value is
not used by the node.

Differential Revision: https://developer.blender.org/D13335