Remove Fetch's same-origin data URL flag

The change to Fetch discussed in
whatwg/fetch#381 made it obsolete.

Closes #1243, closes #1778, and closes #1779 as these are all treated
as same-origin now per the change to Fetch.

source

@@ -2844,7 +2844,6 @@ a.setAttribute('href', 'http://example.com/'); // change the content attribute d
        <li><dfn data-noexport="" data-x="concept-request-priority" data-x-href="https://fetch.spec.whatwg.org/#concept-request-priority">priority</dfn>
        <li><dfn data-noexport="" data-dfn-for="request" data-x="concept-request-origin" data-x-href="https://fetch.spec.whatwg.org/#concept-request-origin">origin</dfn>
        <li><dfn data-noexport="" data-x-href="https://fetch.spec.whatwg.org/#omit-origin-header-flag">omit-<code>Origin</code>-header flag</dfn>
-       <li><dfn data-noexport="" data-x-href="https://fetch.spec.whatwg.org/#same-origin-data-url-flag">same-origin data-URL flag</dfn>
        <li><dfn data-noexport="" data-x="concept-request-referrer" data-x-href="https://fetch.spec.whatwg.org/#concept-request-referrer">referrer</dfn>
        <li><dfn data-noexport="" data-x-href="https://fetch.spec.whatwg.org/#synchronous-flag">synchronous flag</dfn>
        <li><dfn data-noexport="" data-x="concept-request-mode" data-x-href="https://fetch.spec.whatwg.org/#concept-request-mode">mode</dfn>
@@ -26236,8 +26235,6 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</pre>
     <code>picture</code></span>, set <var>request</var>'s <span
     data-x="concept-request-initiator">initiator</span> to "<code data-x="">imageset</code>".</p>
 
-    <p>&#x231B; Set <var>request</var>'s <span>same-origin data-URL flag</span>.</p>
-
     <p>&#x231B; Set <var>request</var>'s
     <span data-x="concept-request-referrer-policy">referrer policy</span> to the current state of
     the element's <code data-x="attr-img-referrerpolicy">referrerpolicy</code> attribute.</p>
@@ -27101,8 +27098,6 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</pre>
      data-x="">imageset</code>", and set <var>request</var>'s <span>synchronous
      flag</span>.</p></li>
 
-     <li><p>Set <var>request</var>'s <span>same-origin data-URL flag</span>.</p></li>
-
      <li><p>Set <var>request</var>'s
      <span data-x="concept-request-referrer-policy">referrer policy</span> to the current state of
      the element's <code data-x="attr-img-referrerpolicy">referrerpolicy</code> attribute.</p></li>