remove NotebookDocument#cells in favor of cellsAt, getCells, and cellCount

src/vs/workbench/api/common/extHostNotebookDocument.ts

@@ -179,7 +179,6 @@ export class ExtHostNotebookDocument extends Disposable {
 				get isUntitled() { return that.uri.scheme === Schemas.untitled; },
 				get isClosed() { return that._disposed; },
 				get metadata() { return that._metadata; },
-				get cells(): ReadonlyArray<vscode.NotebookCell> { return that._cells.map(cell => cell.cell); },
 				get cellCount() { return that._cells.length; },
 				cellAt(index) {
 					index = that._validateIndex(index);