Remove breadcrumbs style from TOC

The negative margin was causing it to overlap the previous header

_includes/full_toc.html

 <h2>Contents</h2>
-  <nav id="full-toc" class="breadcrumb-nav">
+  <nav id="full-toc">
     <ol class="breadcrumb-nav-list">
       {% assign pages_list = site.html_pages | sort:"nav_order" %}
       {% for node in pages_list %}