Remove preview_markdown from the global obj

app/assets/javascripts/behaviors/index.js

@@ -7,6 +7,7 @@ import installGlEmojiElement from './gl_emoji';
 import './quick_submit';
 import './requires_input';
 import './toggler_behavior';
+import '../preview_markdown';
 
 installGlEmojiElement();
 initCopyAsGFM();

app/assets/javascripts/main.js

@@ -46,7 +46,6 @@ import LazyLoader from './lazy_loader';
 import './line_highlighter';
 import initLogoAnimation from './logo';
 import './milestone_select';
-import './preview_markdown';
 import './projects_dropdown';
 import './render_gfm';
 import initBreadcrumbs from './breadcrumb';