message: remove unused ContextApi
diff: @@ -14,7 +14,7 @@ use storage::{BlockHeaderWithHash, BlockStorage, BlockStorageReader, ContextActi
  use storage::block_storage::BlockJsonData;
  use storage::persistent::PersistentStorage;
  use storage::skip_list::Bucket;
  -use storage::context::{TezedgeContext, ContextIndex, ContextApi};
  +use storage::context::{TezedgeContext, ContextIndex};
  use tezos_context::channel::ContextAction;
  use tezos_messages::protocol::{RpcJsonMap, UniversalValue};
  