🚮 Remove src/purifier.js rollup config

[samouri]

summary
Removes the rollup config for src/purifier since we have no intention of distributing it as a library.

• renamed src/purifier/purifier.js --> src/purifier/index.js
• updated import to point to #purifier instead of #purifier/purifier

[rcebulko]

amphtml/src/purifier/README.md:
    9  
   10  ```js
   11: import {Purifier} from './src/purifier/purifier';
   12  
   13  const purifier = new Purifier(document);
   ..
   19  ```js
   20  import Mustache from 'mustache';
   21: import {Purifier} from '../../../src/purifier/purifier';
   22  
   23  const purifier = new Purifier(document);

amphtml/test/unit/test-purifier.js:
   16  
   17  import * as urlRewrite from '../../src/url-rewrite';
   18: import {Purifier} from '../../src/purifier/purifier';
   19  
   20  describes.sandboxed

amphtml/build-system/test-configs/dep-check-config.js:
   44        // WARNING: Importing purifier.js will also bundle DOMPurify (13KB).
   45        'extensions/amp-list/0.1/amp-list.js->src/purifier/sanitation.js',
   46:       'extensions/amp-mustache/0.2/amp-mustache.js->src/purifier/purifier.js',
   47:       'extensions/amp-script/0.1/amp-script.js->src/purifier/purifier.js',
   48:       'src/purifier/purifier.js->src/purifier/sanitation.js',
   49        'src/sanitizer.js->src/purifier/sanitation.js',
   50      ],

amphtml/extensions/amp-script/0.1/amp-script.js:
   19  import {Deferred} from '../../../src/core/data-structures/promise';
   20  import {Layout, isLayoutSizeDefined} from '../../../src/layout';
   21: import {Purifier} from '../../../src/purifier/purifier';
   22  import {Services} from '../../../src/services';
   23  import {UserActivationTracker} from './user-activation-tracker';

amphtml/extensions/amp-mustache/0.2/amp-mustache.js:
   16  
   17  import {BaseTemplate} from '../../../src/base-template';
   18: import {Purifier} from '../../../src/purifier/purifier';
   19  import {dict} from '../../../src/core/types/object';
   20  import {getService, registerServiceBuilder} from '../../../src/service';

amphtml/extensions/amp-mustache/0.2/test/test-amp-mustache.js:
   17  import * as service from '../../../../src/service';
   18  import {AmpMustache} from '../amp-mustache';
   19: import {Purifier} from '../../../../src/purifier/purifier';
   20  import mustache from '../../../../third_party/mustache/mustache';
   21  

[rcebulko]

chef's kiss