refactor(compiler): Drop support for the deprecated <template>. Use <ng-template> instead (#22783)

BREAKING CHANGE:

The `<template>` tag was deprecated in Angular v4 to avoid collisions (i.e. when
using Web Components).

This commit removes support for `<template>`. `<ng-template>` should be used
instead.

BEFORE:

    <!-- html template -->
    <template>some template content</template>

    # tsconfig.json
    {
      # ...
      "angularCompilerOptions": {
        # ...
        # This option is no more supported and will have no effect
        "enableLegacyTemplate": [true|false]
      }
    }

AFTER:

    <!-- html template -->
    <ng-template>some template content</ng-template>

PR Close #22783

packages/compiler-cli/src/ngtools_api2.ts

@@ -50,7 +50,6 @@ export interface CompilerOptions extends ts.CompilerOptions {
   annotateForClosureCompiler?: boolean;
   annotationsAs?: 'decorators'|'static fields';
   trace?: boolean;
-  enableLegacyTemplate?: boolean;
   disableExpressionLowering?: boolean;
   i18nOutLocale?: string;
   i18nOutFormat?: string;

packages/compiler-cli/src/transformers/api.ts

@@ -125,9 +125,6 @@ export interface CompilerOptions extends ts.CompilerOptions {
   // Print extra information while running the compiler
   trace?: boolean;
 
-  // Whether to enable support for <template> and the template attribute (false by default)
-  enableLegacyTemplate?: boolean;
-
   // Whether to enable lowering expressions lambdas and expressions in a reference value
   // position.
   disableExpressionLowering?: boolean;

packages/compiler-cli/src/transformers/program.ts

@@ -828,7 +828,6 @@ function getAotCompilerOptions(options: CompilerOptions): AotCompilerOptions {
   return {
     locale: options.i18nInLocale,
     i18nFormat: options.i18nInFormat || options.i18nOutFormat, translations, missingTranslation,
-    enableLegacyTemplate: options.enableLegacyTemplate,
     enableSummariesForJit: options.enableSummariesForJit,
     preserveWhitespaces: options.preserveWhitespaces,
     fullTemplateTypeCheck: options.fullTemplateTypeCheck,