Remove deprecated external.py module

External tables (~external_*) are deprecated in favor of the new
AttributeType-based storage system. The new types (<xblob>, <content>,
<object>) store data directly to storage via StorageBackend without
tracking tables.

- Remove src/datajoint/external.py entirely
- Remove ExternalMapping from schemas.py
- Remove external table pre-declaration from table.py

Co-authored-by: dimitri-yatsenko <dimitri@datajoint.com>

Author: claude