Removed unused externalDatabase Helm chart configuration (#388)

## What
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* Describe what the change is solving.
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Issue: airbytehq/airbyte#54176

Current Helm `values.yml` documentation and defaults have old unused
values for `externalDatabase`, potentially leading to confusion when
setting up or upgrading Helm chart versions.

## How
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* Describe how code changes achieve the solution.
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This removes `externalDatabase` from all defaults and documentation.
These values are not used anywhere in the project outside of the changes
files in this PR. Instead `global.database` should be used instead to
configure external PostgreSQL connections.

## Can this PR be safely reverted and rolled back?
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* If that is not the case (e.g. a database migration), check NO.
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- [x] YES 💚
- [ ] NO ❌

Co-authored-by: perangel <[email protected]>

Author: Tybot204