remove unused optional portion of CollectFieldsContext

this was from earlier iteration

Author: yaacovCR

src/execution/collectFields.ts

@@ -56,7 +56,6 @@ interface CollectFieldsContext {
   schema: GraphQLSchema;
   fragments: ObjMap<FragmentDetails>;
   variableValues: { [variable: string]: unknown };
-  fragmentVariableValues?: FragmentVariables;
   operation: OperationDefinitionNode;
   runtimeType: GraphQLObjectType;
   visitedFragmentNames: Set<string>;