feat(compiler): set enableLegacyTemplate to false by default (#18756)

BREAKING CHANGE: the compiler option `enableLegacyTemplate` is now disabled by default as the `<template>` element has been deprecated since v4. Use `<ng-template>` instead. The option `enableLegacyTemplate` and the `<template>` element will both be removed in Angular v6.
PR Close #18756

packages/common/src/directives/ng_if.ts

@@ -75,7 +75,6 @@ import {Directive, EmbeddedViewRef, Input, TemplateRef, ViewContainerRef} from '
  *
  * Simple form:
  * - `<div *ngIf="condition">...</div>`
- * - `<div template="ngIf condition">...</div>`
  * - `<ng-template [ngIf]="condition"><div>...</div></ng-template>`
  *
  * Form with an else block: