go/runtime/registry: Remove runtime host handler factory

[peternose]

Refactored committee node:

• Removed RuntimeHostHandlerFactory as it is more cleaner to pass parameters to the constructor instead of passing a factory.
• Removed handler and notifier from the committee node, so that we can easier stop/start the notifier.
• Removed host provisioner (which is now generalized to provision any runtime) from the runtime registry, as it doesn't belong there.
  • It is now globally avaliable to common worker which provisions runtimes and to the key manager worker which provisions key manager runtime (maybe we need to move up also handler and notifier).

Related to:

• Decouple ROFL event notifier and sub-handler #6014

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