[ref] gva: remove mutation

lib/modules/gva/resources/schema.gql

 
 schema {
   query: Query
-  mutation: Mutation
 }
 
 type Query {
   current: Block @juniper(ownership: "owned")
 }
 
-type Mutation 
-
 type Block {
   version: Int!
   currency: String!

lib/modules/gva/src/schema.rs

@@ -19,6 +19,7 @@ use crate::context::Context;
 use dubp_block_doc::block::BlockDocumentTrait;
 use dubp_common_doc::traits::Document;
 use durs_bc_db_reader::BcDbRo;
+use juniper::EmptyMutation;
 use juniper::Executor;
 use juniper::FieldResult;
 use juniper_from_schema::graphql_schema_from_file;
@@ -86,12 +87,6 @@ impl BlockFields for Block {
     }
 }
 
-pub struct Mutation;
-
-impl MutationFields for Mutation {
-
-}
-
 pub fn create_schema() -> Schema {
-    Schema::new(Query {}, Mutation {})
+    Schema::new(Query {}, EmptyMutation::new())
 }