single-src: remove _production field

Author: kaiosilveira

src/single-source/index.js

@@ -1,7 +1,6 @@
 export class ProductionPlan {
   constructor() {
     this._adjustments = [];
-    this._production = 0;
   }
 
   get production() {
@@ -10,6 +9,5 @@ export class ProductionPlan {
 
   applyAdjustment(anAdjustment) {
     this._adjustments.push(anAdjustment);
-    this._production += anAdjustment.amount;
   }
 }