PLT-5913: Remove id from heading markdown compilation (#5782)

Id for heading markdown is unnecessary as we didn't
attach any style/effect on that id, and it also introduce
duplicate id. So remove it from heading markdown generatiing.

webapp/tests/formatting_hashtags.test.jsx

@@ -11,7 +11,7 @@ describe('TextFormatting.Hashtags', function() {
     it('Not hashtags', function(done) {
         assert.equal(
             TextFormatting.formatText('# hashtag').trim(),
-            '<h1 id="hashtag" class="markdown__heading">hashtag</h1>'
+            '<h1 class="markdown__heading">hashtag</h1>'
         );
 
         assert.equal(

webapp/utils/markdown.jsx

@@ -156,9 +156,8 @@ class MattermostMarkdownRenderer extends marked.Renderer {
         return out;
     }
 
-    heading(text, level, raw) {
-        const id = `${this.options.headerPrefix}${raw.toLowerCase().replace(/[^\w]+/g, '-')}`;
-        return `<h${level} id="${id}" class="markdown__heading">${text}</h${level}>`;
+    heading(text, level) {
+        return `<h${level} class="markdown__heading">${text}</h${level}>`;
     }
 
     link(href, title, text) {