Just posting a fix here in case anyone finds ‘Description’ fields have disappeared across the admin screens. For me it interfered with Ajax Event Calendar and i lost the description fields in the options and add event areas.
At line 98 of categorytinymce.php i have adapted the style rules to scope to the description field of the category edit page.
<style type="text/css">
.quicktags-toolbar input {float:left !important; width:auto !important;}
/**/.taxonomy-category .term-description-wrap [for="description"], .taxonomy-category .term-description-wrap textarea#description, .term-description-wrap p.description {display:none!important;}
</style>
https://wordpress.org/plugins/categorytinymce/