[CHORE] Drop unused __about__.py files from Python packages

[lowlydba]

Context

Follow-up from review discussion on #534: #534 (comment)

__about__.py currently exists in each Python package to expose __version__ programmatically:

• packages/overture-schema/src/overture/schema/__about__.py
• packages/overture-schema-addresses-theme/src/overture/schema/addresses/__about__.py
• packages/overture-schema-annex/src/overture/schema/__about__.py
• packages/overture-schema-base-theme/src/overture/schema/base/__about__.py
• packages/overture-schema-buildings-theme/src/overture/schema/buildings/__about__.py
• packages/overture-schema-cli/src/overture/schema/cli/__about__.py
• packages/overture-schema-codegen/src/overture/schema/codegen/__about__.py
• packages/overture-schema-common/src/overture/schema/common/__about__.py
• packages/overture-schema-divisions-theme/src/overture/schema/divisions/__about__.py
• packages/overture-schema-places-theme/src/overture/schema/places/__about__.py
• packages/overture-schema-system/src/overture/schema/system/__about__.py
• packages/overture-schema-transportation-theme/src/overture/schema/transportation/__about__.py

Per @sethfitz's comment:

__version__ was in code to be programmatically accessible, but I didn't realize that importlib.metadata had a good way to access it. Given that, I'd be inclined to drop __about__.py from all of the packages. AFAIK, nothing references __version__.

Task

• Confirm nothing in the codebase (or downstream consumers) references __version__ from these __about__.py files.
• Remove __about__.py from all packages listed above.
• Update any package configuration (e.g. pyproject.toml version source/hatch config) that currently points at these files to use importlib.metadata or another equivalent mechanism instead.