Simplify header DOM, style it according to specs

[MAILPOET-2777]

assets/js/src/common/modal/header.jsx

@@ -5,11 +5,9 @@ import Heading from 'common/typography/heading/heading';
 
 const ModalHeader = ({ title }) => (
   <div className="mailpoet-modal-header">
-    <div className="mailpoet-modal-header-heading-container">
-      <Heading level={3}>
-        { title }
-      </Heading>
-    </div>
+    <Heading level={3}>
+      { title }
+    </Heading>
   </div>
 );