Remove severity field from all rules

The severity rating is no longer used. This removes:
- The severity front matter field from all 130 rule files
- The severity icon <img> from the rule-category layout
- The unused severity images (1.png, 2.png, 3.png)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: dennisdoomen

_layouts/rule-category.html

@@ -7,6 +7,6 @@
     {% if rule.custom_prefix != nil  %}
         {% assign full_rule_id = rule.custom_prefix | append: rule.rule_id  %}
     {% endif %}
-    <h3 id="{{ full_rule_id }}">{{ rule.title | markdownify | replace: "<p>", "" | replace: "</p>", "" }} ({{ full_rule_id }}) <img src="/assets/images/{{ rule.severity }}.png" /></h3>
+    <h3 id="{{ full_rule_id }}">{{ rule.title | markdownify | replace: "<p>", "" | replace: "</p>", "" }} ({{ full_rule_id }})</h3>
     {{ rule.content }}
 {% endfor %}