Deprecate IfcExternalSpatialStructureElement and subtype

Description of the proposal:

Describe how it contributes to the objectives set in https://github.com/buildingSMART/NextGen-IFC/wiki/Towards-a-technology-independent-IFC:

What do we win:
Spatial Structure element that has not been used.
What do we loose
nothing
Schema impact:
low
Instance model impact:
non existant
Backwards compatible:
yes
Automatic migration possible:
yes
Additional implications:
can be modeled with IfcSpace.EXTERNAL at IfcSpace level and IfcFacility and IfcFacilityPart on higher levels
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Note that not all points need to be satisfied!
Backwards compatibility and file size are not concerns.

Read the proposal and the linked “Towards a technology-independent IFC” wiki page, then locate the schema definitions for IfcExternalSpatialStructureElement and its subtype. Confirm how the proposed removal relates to IfcSpace.EXTERNAL, IfcFacility, and IfcFacilityPart, and document whether the schema and instance-model impacts match the issue before marking the deprecation complete.

Written by the indexing model from the issue text.