🔥 remove unused markdown/preview helper and component

no issue
- `gh-format-markdown` is no longer used
- `gh-ed-preview` is a hangover from the old editor

app/helpers/gh-format-markdown.js

@@ -1,33 +0,0 @@
-/* global Showdown, html_sanitize */
-import {helper} from 'ember-helper';
-import {htmlSafe} from 'ember-string';
-import cajaSanitizers from 'ghost-admin/utils/caja-sanitizers';
-
-// eslint-disable-next-line new-cap
-let showdown = new Showdown.converter({extensions: ['ghostimagepreview', 'ghostgfm', 'footnotes', 'highlight']});
-
-export function formatMarkdown(params) {
-    if (!params || !params.length) {
-        return;
-    }
-
-    let markdown = params[0] || '';
-    let escapedhtml = '';
-
-    // convert markdown to HTML
-    escapedhtml = showdown.makeHtml(markdown);
-
-    // replace script and iFrame
-    escapedhtml = escapedhtml.replace(/<script\b[^<]*(?:(?!<\/script>)<[^<]*)*<\/script>/gi,
-        '<pre class="js-embed-placeholder">Embedded JavaScript</pre>');
-    escapedhtml = escapedhtml.replace(/<iframe\b[^<]*(?:(?!<\/iframe>)<[^<]*)*<\/iframe>/gi,
-        '<pre class="iframe-embed-placeholder">Embedded iFrame</pre>');
-
-    // sanitize html
-    /* eslint-disable-next-line camelcase */
-    escapedhtml = html_sanitize(escapedhtml, cajaSanitizers.url, cajaSanitizers.id);
-
-    return htmlSafe(escapedhtml);
-}
-
-export default helper(formatMarkdown);

app/templates/components/gh-ed-preview.hbs

@@ -1,13 +0,0 @@
-{{previewHTML}}
-
-{{#each imageUploadComponents as |uploader|}}
-    {{#ember-wormhole to=uploader.destinationElementId}}
-        {{gh-image-uploader-with-preview
-            image=uploader.src
-            altText=uploader.altText
-            update=(action "updateImageSrc" uploader.index)
-            remove=(action "updateImageSrc" uploader.index "")
-            uploadStarted=uploadStarted
-            uploadFinished=uploadFinished}}
-    {{/ember-wormhole}}
-{{/each}}