Remove mutability check from legacy activation behaviors

This is a followup to whatwg#5805, removing some now-redundant checks while
fixing the condition introduced there to be a bit less far-reaching.
(In particular, we want disabled checkboxes and radio buttons to still
be immutable, even if that doesn't impact their activation behavior.)

source

@@ -44700,9 +44700,8 @@ interface <dfn>HTMLInputElement</dfn> : <span>HTMLElement</span> {
   agent should not allow the user to modify the element's <span
   data-x="concept-fe-value">value</span> or <span data-x="concept-fe-checked">checkedness</span>.</p>
 
-  <p>When an <code>input</code> element is <span data-x="concept-fe-disabled">disabled</span> and
-  is not in the <span data-x="attr-input-type-checkbox">Checkbox</span> nor <span
-  data-x="attr-input-type-radio">Radio</span> states, it is not <i
+  <p>When an <code>input</code> element is <span
+  data-x="concept-fe-disabled">disabled</span>, it is not <i
   data-x="concept-fe-mutable">mutable</i>.</p>
 
   <p class="note">The <code data-x="attr-input-readonly">readonly</code> attribute can also in some
@@ -44720,9 +44719,9 @@ interface <dfn>HTMLInputElement</dfn> : <span>HTMLElement</span> {
   <p>The <span>activation behavior</span> for <code>input</code> elements are these steps:</p>
 
   <ol>
-   <li><p>If this element is not <i data-x="concept-fe-mutable">mutable</i> and is not in <span
-   data-x="attr-input-type-checkbox">Checkbox</span> nor <span data-x="attr-input-type-radio">
-   Radio</span>, then return.</p></li>
+   <li><p>If this element is not <i data-x="concept-fe-mutable">mutable</i> and is not in the <span
+   data-x="attr-input-type-checkbox">Checkbox</span> state and is not in the <span
+   data-x="attr-input-type-radio">Radio</span> state, then return.</p></li>
 
    <li><p>Run this element's <dfn>input activation behavior</dfn>, if any, and do nothing
    otherwise.</p></li>
@@ -44732,8 +44731,6 @@ interface <dfn>HTMLInputElement</dfn> : <span>HTMLElement</span> {
   steps:</p>
 
   <ol>
-   <li><p>If this element is not <i data-x="concept-fe-mutable">mutable</i>, then return.</p></li>
-
    <li><p>If this element's <code data-x="attr-input-type">type</code> attribute is in the <span
    data-x="attr-input-type-checkbox">Checkbox state</span>, then set this element's <span
    data-x="concept-fe-checked">checkedness</span> to its opposite value (i.e. true if it is false,
@@ -44751,8 +44748,6 @@ interface <dfn>HTMLInputElement</dfn> : <span>HTMLElement</span> {
   steps:</p>
 
   <ol>
-   <li><p>If the element is not <i data-x="concept-fe-mutable">mutable</i>, then return.</p></li>
-
    <li><p>If the element's <code data-x="attr-input-type">type</code> attribute is in the <span
    data-x="attr-input-type-checkbox">Checkbox state</span>, then set the element's <span
    data-x="concept-fe-checked">checkedness</span> and the element's <code