fix(@schematics/angular): remove explicit index option from new applications

With the index option now defaulting to `<project_source_root>/index.html`
for the `application` build system, the explicit value present in new
applications is no longer required. This removal further reduces the size
of the `angular.json` file for new projects.

Author: clydin

packages/schematics/angular/application/index.ts

@@ -256,7 +256,6 @@ function addAppToWorkspaceFile(
         builder: Builders.BuildApplication,
         defaultConfiguration: 'production',
         options: {
-          index: `${sourceRoot}/index.html`,
           browser: `${sourceRoot}/main.ts`,
           polyfills: options.experimentalZoneless ? [] : ['zone.js'],
           tsConfig: `${projectRoot}tsconfig.app.json`,

packages/schematics/angular/application/index_spec.ts

@@ -314,7 +314,7 @@ describe('Application Schematic', () => {
       const prj = config.projects.foo;
       expect(prj.root).toEqual('');
       const buildOpt = prj.architect.build.options;
-      expect(buildOpt.index).toEqual('src/index.html');
+      expect(buildOpt.index).toBeUndefined();
       expect(buildOpt.browser).toEqual('src/main.ts');
       expect(buildOpt.assets).toEqual([{ 'glob': '**/*', 'input': 'public' }]);
       expect(buildOpt.polyfills).toEqual(['zone.js']);
@@ -405,7 +405,6 @@ describe('Application Schematic', () => {
       const project = config.projects.foo;
       expect(project.root).toEqual('foo');
       const buildOpt = project.architect.build.options;
-      expect(buildOpt.index).toEqual('foo/src/index.html');
       expect(buildOpt.browser).toEqual('foo/src/main.ts');
       expect(buildOpt.polyfills).toEqual(['zone.js']);
       expect(buildOpt.tsConfig).toEqual('foo/tsconfig.app.json');