Move heading styles from CSS file to theme.json

Because we want to have all editor configurations in theme.json, I moved heading font sized to theme.json and removed redundant filters.
[MAILPOET-5640]

mailpoet/lib/EmailEditor/Engine/SettingsController.php

@@ -73,8 +73,6 @@ class SettingsController {
       ['css' => $flexEmailLayoutStyles],
     ];
 
-    $settings['styles'] = apply_filters('mailpoet_email_editor_editor_styles', $settings['styles']);
-
     $settings['__experimentalFeatures'] = $themeSettings;
 
     // Enabling alignWide allows full width for specific blocks such as columns, heading, image, etc.