Remove collation-generation subsystem from validator nodes (#6832)

# Description
Issue #6476 
Collation-generation is not needed for validators node, and should be
removed.

## Implementation
Use a `DummySubsystem` for `collation_generation`

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Co-authored-by: Bastian Köcher <[email protected]>
Co-authored-by: command-bot <>
Co-authored-by: Dmitry Markin <[email protected]>
Co-authored-by: Alexandru Vasile <[email protected]>