Revert "PLT-3181 Added crossorigin='anonymous' to all external images (#5774)" (#5815)

This reverts commit 24848f9d6a.

webapp/utils/markdown.jsx

@@ -152,7 +152,6 @@ class MattermostMarkdownRenderer extends marked.Renderer {
             out += ' height="' + dimensions[1] + '"';
         }
         out += ' onload="window.markdownImageLoaded(this)" onerror="window.markdownImageLoaded(this)" class="markdown-inline-img"';
-        out += ' crossorigin="anonymous"';
         out += this.options.xhtml ? '/>' : '>';
         return out;
     }