Remove dangling "fallback width" and "width" references

...from the "Updating the source set" algorithm.

Mop-up for Issue #5893, completing work started in PR #5900.

source

@@ -29003,9 +29003,8 @@ was an English &lt;a href="/wiki/Music_hall">music hall&lt;/a> singer, ...</code
      its value does not <span data-x="matches the environment">match the environment</span>,
      continue to the next child.</p></li>
 
-     <li><p><span data-x="parse a sizes attribute">Parse <var>child</var>'s sizes attribute</span>
-     with the fallback width <var>width</var>, and let <var>source set</var>'s <span>source
-     size</span> be the returned value.</p></li>
+     <li><p><span data-x="parse a sizes attribute">Parse <var>child</var>'s sizes attribute</span>,
+     and let <var>source set</var>'s <span>source size</span> be the returned value.</p></li>
 
      <li><p>If <var>child</var> has a <code data-x="attr-source-type">type</code> attribute, and its
      value is an unknown or unsupported <span>MIME type</span>, continue to the next child.</p></li>