Remove ingestr-based ctk load examples

[hammerhead]

About

Building this project started failing with:

/home/runner/work/cratedb-guide/cratedb-guide/docs/integrate/mysql/usage.md:5: WARNING: undefined label: 'ctk:mysql' [ref.ref]
/home/runner/work/cratedb-guide/cratedb-guide/docs/integrate/oracle/usage.md:4: WARNING: undefined label: 'ctk:oracle' [ref.ref]
/home/runner/work/cratedb-guide/cratedb-guide/docs/integrate/postgresql/usage.md:4: WARNING: undefined label: 'ctk:postgresql' [ref.ref]

The Oracle and PostgreSQL sections were entirely based on ctk load. The MySQL one still had a generic CSV export/import case covered, which I kept.

Preview

https://cratedb-guide--605.org.readthedocs.build/integrate/index.html

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[bgunebakan]

Looks good, docs/ingest/etl/index.md Line: 32-33 still reference this deleted pages, Should we also delete them?