Cleanup bolierplate code when DisabledValidators runtime api call is released

[tdimitrov]

When DisabledValidators runtime api is released (atm at target version 8) remove the boilerplate code from:

• backing

  polkadot-sdk/polkadot/node/core/backing/src/lib.rs

  Lines 1029 to 1052 in d7f4315

  	// TODO: https://github.com/paritytech/polkadot-sdk/issues/1940
  	// Once runtime ver `DISABLED_VALIDATORS_RUNTIME_REQUIREMENT` is released remove this `if`
  	// statement, add `request_from_runtime` call to the `try_join!` call above and use
  	// `try_runtime_api!` to get `disabled_validators`
  	let disabled_validators = if has_required_runtime(
  	ctx.sender(),
  	parent,
  	RuntimeApiRequest::DISABLED_VALIDATORS_RUNTIME_REQUIREMENT,
  	)
  	.await
  	{
  	let disabled_validators = request_from_runtime(parent, ctx.sender(), |tx| {
  	RuntimeApiRequest::DisabledValidators(tx)
  	})
  	.await
  	.await
  	.map_err(Error::JoinMultiple)?;
  	let disabled_validators = try_runtime_api!(disabled_validators);
  	disabled_validators
  	} else {
  	gum::debug!(target: LOG_TARGET, "Runtime doesn't support `DisabledValidators` - continuing with an empty disabled validators set");
  	vec![]
  	};

• subsystem-util

  polkadot-sdk/polkadot/node/subsystem-util/src/lib.rs

  Lines 461 to 477 in d7f4315

  	// TODO: https://github.com/paritytech/polkadot-sdk/issues/1940
  	// When `DisabledValidators` is released remove this `if`` and add a
  	// `request_disabled_validators` call in the `try_join!` call above
  	let disabled_validators = if has_required_runtime(
  	sender,
  	parent,
  	RuntimeApiRequest::DISABLED_VALIDATORS_RUNTIME_REQUIREMENT,
  	)
  	.await
  	{
  	request_disabled_validators(parent, sender).await.await??
  	} else {
  	vec![]
  	};
  	Self::construct(&validators, &disabled_validators, signing_context, keystore)
  	}

• helper in subsystem-util/vstaging

  polkadot-sdk/polkadot/node/subsystem-util/src/vstaging.rs

  Lines 30 to 56 in 15b5069

  	/// Returns disabled validators list if the runtime supports it. Otherwise logs a debug messages and
  	/// returns an empty vec.
  	/// Once runtime ver `DISABLED_VALIDATORS_RUNTIME_REQUIREMENT` is released remove this function and
  	/// replace all usages with `request_disabled_validators`
  	/// TODO: https://github.com/paritytech/polkadot-sdk/issues/1940
  	pub async fn get_disabled_validators_with_fallback<Sender: SubsystemSender<RuntimeApiMessage>>(
  	sender: &mut Sender,
  	relay_parent: Hash,
  	) -> Result<Vec<ValidatorIndex>, Error> {
  	let disabled_validators = if has_required_runtime(
  	sender,
  	relay_parent,
  	RuntimeApiRequest::DISABLED_VALIDATORS_RUNTIME_REQUIREMENT,
  	)
  	.await
  	{
  	request_disabled_validators(relay_parent, sender)
  	.await
  	.await
  	.map_err(Error::Oneshot)??
  	} else {
  	gum::debug!(target: LOG_TARGET, "Runtime doesn't support `DisabledValidators` - continuing with an empty disabled validators set");
  	vec![]
  	};
  	Ok(disabled_validators)
  	}