Revert "Preserve object prototypes when cloning (chartjs#7404)"

This reverts commit 484f0d1.

src/helpers/helpers.core.js

@@ -179,7 +179,7 @@ var helpers = {
 		}
 
 		if (helpers.isObject(source)) {
-			var target = Object.create(source);
+			var target = {};
 			var keys = Object.keys(source);
 			var klen = keys.length;
 			var k = 0;

test/specs/helpers.core.tests.js

@@ -306,25 +306,6 @@ describe('Chart.helpers.core', function() {
 			expect(output.o.a).not.toBe(a1);
 			expect(output.a).not.toBe(a0);
 		});
-		it('should preserve prototype of objects', function() {
-			// https://github.com/chartjs/Chart.js/issues/7340
-			function MyConfigObject(s) {
-				this._s = s;
-			}
-			MyConfigObject.prototype.func = function() {
-				return 10;
-			};
-			var original = new MyConfigObject('something');
-			var output = helpers.merge({}, {
-				plugins: [{
-					test: original
-				}]
-			});
-			var clone = output.plugins[0].test;
-			expect(clone).toBeInstanceOf(MyConfigObject);
-			expect(clone).toEqual(original);
-			expect(clone === original).toBeFalse();
-		});
 	});
 
 	describe('merge', function() {