sc-cli: Remove SubstrateCli::native_runtime_version function (#14511)

The native runtime will be removed in the near future and thus this function will not be required
anymore.

\# Code changes

Downstream users just need to remove `native_runtime_version` from their implementation of the
`SubstrateCli` trait.

substrate/client/cli/src/commands/insert_key.rs

@@ -125,10 +125,6 @@ mod tests {
 			"test".into()
 		}
 
-		fn native_runtime_version(_: &Box<dyn ChainSpec>) -> &'static sp_version::RuntimeVersion {
-			unimplemented!("Not required in tests")
-		}
-
 		fn load_spec(&self, _: &str) -> std::result::Result<Box<dyn ChainSpec>, String> {
 			Ok(Box::new(GenericChainSpec::from_genesis(
 				"test",