Merge txpool-runtime-api with txpool-api (#4320)

* Remove transaction-pool-runtime-api

* Merge runtime-api with transaction-pool.

substrate/client/rpc/src/author/mod.rs

@@ -39,7 +39,7 @@ use sp_api::ConstructRuntimeApi;
 use sp_runtime::{generic, traits::{self, ProvideRuntimeApi}};
 use txpool_api::{
 	TransactionPool, InPoolTransaction, TransactionStatus,
-	BlockHash, TxHash, TransactionFor, IntoPoolError,
+	BlockHash, TxHash, TransactionFor, error::IntoPoolError,
 };
 use session::SessionKeys;