refactor(@angular-devkit/schematics): remove UpdateBuffer and rename UpdateBuffer2 to UpdateBuffer

This PR removes the internally built `UpdateBuffer` and renames
`UpdateBuffer2` (based on magic-string) to `UpdateBuffer`. This
should have little to no impact for consumers.

BREAKING CHANGE:
The depracated `UpdateBuffer` has been removed and `UpdateBuffer2`
is renamed to `UpdateBuffer`. With this change the related and
deprecated symbols `ContentCannotBeRemovedException` and `Chunk`
have also been removed.

Author: kyubisation

packages/angular_devkit/schematics/src/tree/recorder_spec.ts

@@ -7,7 +7,6 @@
  */
 
 import { normalize } from '@angular-devkit/core';
-import { UpdateBuffer2, UpdateBufferBase } from '../utility/update-buffer';
 import { SimpleFileEntry } from './entry';
 import { UpdateRecorderBase, UpdateRecorderBom } from './recorder';
 
@@ -36,11 +35,6 @@ describe('UpdateRecorderBase', () => {
     const buffer = Buffer.from('Hello beautiful World');
     const entry = new SimpleFileEntry(normalize('/some/path'), buffer);
 
-    // TODO: Remove once UpdateBufferBase.create defaults to UpdateBuffer2
-    spyOn(UpdateBufferBase, 'create').and.callFake(
-      (originalContent) => new UpdateBuffer2(originalContent),
-    );
-
     const recorder = new UpdateRecorderBase(entry);
     recorder.remove(6, 9);
     recorder.insertRight(6, 'amazing');