Remove unused getRingIndex from Doughnut (#8521)

Author: kurkle

docs/docs/getting-started/v3-migration.md

@@ -303,6 +303,7 @@ The following properties and methods were removed:
 * `DatasetController.addElementAndReset`
 * `DatasetController.createMetaData`
 * `DatasetController.createMetaDataset`
+* `DoughnutController.getRingIndex`
 
 #### Removed from Elements
 

src/controllers/controller.doughnut.js

@@ -60,19 +60,6 @@ export default class DoughnutController extends DatasetController {
     }
   }
 
-  // Get index of the dataset in relation to the visible datasets. This allows determining the inner and outer radius correctly
-  getRingIndex(datasetIndex) {
-    let ringIndex = 0;
-
-    for (let j = 0; j < datasetIndex; ++j) {
-      if (this.chart.isDatasetVisible(j)) {
-        ++ringIndex;
-      }
-    }
-
-    return ringIndex;
-  }
-
   /**
 	 * @private
 	 */

types/index.esm.d.ts

@@ -314,7 +314,6 @@ export interface DoughnutController extends DatasetController {
 	readonly offsetX: number;
 	readonly offsetY: number;
 
-	getRingIndex(datasetIndex: number): number;
 	calculateTotal(): number;
 	calculateCircumference(value: number): number;
 }