Fix unloading KNX integration without sensors

[farmio]

Proposed change

Fix unloading KNX integration without sensors in YAML config.

Since platforms to unload were checked by their appearance in YAML, when there were no sensors configured, the unload was not forwarded to the sensor platform, leaving the system sensors (not configured by YAML) loaded.
This resulted in ValueError: Config entry has already been setup! on a reload.

Sensor platform is always loaded. See https://github.com/home-assistant/core/blob/dev/homeassistant/components/knx/__init__.py#L271-L280

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes KNX Error: Config entry has already been setup #97672
• This PR is related to issue:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
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[home-assistant]

Hey there @Julius2342, @marvin-w, mind taking a look at this pull request as it has been labeled with an integration (knx) you are listed as a code owner for? Thanks!

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[marvin-w]

Nice catch! Thanks!