Remove unimplemented feature forceSpellCheck

I was the one who proposed it, and I think it does deserve to exist, but
it was specced almost five years ago (4a0651c) and no one has
implemented it, so it's time to say goodbye.

source

@@ -9941,7 +9941,6 @@ interface <dfn>HTMLElement</dfn> : <span>Element</span> {
   readonly attribute DOMString <span data-x="dom-accessKeyLabel">accessKeyLabel</span>;
   [<span>CEReactions</span>] attribute boolean <span data-x="dom-draggable">draggable</span>;
   [<span>CEReactions</span>] attribute boolean <span data-x="dom-spellcheck">spellcheck</span>;
-  void <span data-x="dom-forceSpellCheck">forceSpellCheck</span>();
 
   [<span>CEReactions</span>] attribute [TreatNullAs=EmptyString] DOMString <span data-x="dom-innerText">innerText</span>;
 };
@@ -74330,17 +74329,6 @@ body { display:none }
 
    </dd>
 
-
-   <dt><var>element</var> . <code subdfn data-x="dom-forceSpellCheck">forceSpellCheck</code>()</dt>
-
-   <dd>
-
-    <p>Forces the user agent to report spelling and grammar errors on the element (if checking is
-    enabled), even if the user has never focused the element. (If the method is not invoked, user
-    agents can hide errors in text that wasn't just entered by the user.)</p>
-
-   </dd>
-
   </dl>
 
   <div w-nodev>
@@ -74456,15 +74444,6 @@ body { display:none }
   <p>If checking is disabled, the user agent should not indicate spelling or grammar errors for that
   text.</p>
 
-  <p>Even when checking is enabled, user agents may opt to not report spelling or grammar errors in
-  text that the user agent deems the user has no interest in having checked (e.g. text that was
-  already present when the page was loaded, or that the user did not type, or text in controls that
-  the user has not focused, or in parts of e-mail addresses that the user agent is not confident
-  were misspelt). The <dfn><code data-x="dom-forceSpellCheck">forceSpellCheck()</code></dfn> method,
-  when invoked on an element, must override this behavior, forcing the user agent to consider all
-  spelling and grammar errors in text in that element for which checking is enabled to be of
-  interest to the user.</p>
-
   <div class="example">
 
    <p>The element with ID "a" in the following example would be the one used to determine if the