[css-values-5] Remove chapter on attr() cycles (w3c#12009)

This is now specified more generally through "substitution contexts".

Co-authored-by: Anders Hartvoll Ruud <andruud@google.com>

Author: andruud

css-values-5/Overview.bs

@@ -1902,69 +1902,6 @@ Security</h4>
 	''--foo: attr(foo type(&lt;number&gt;)); background-image: src(string(var(--foo)))''
 	needs to be invalid as well.
 
-
-<h4 id=attr-cycles>
-Cycles</h4>
-
-An attribute value may contain ''attr()'' functions,
-or other [=arbitrary substitution functions=],
-that are substituted during [=parse with a <syntax>=].
-This can cause cyclic dependecies,
-either by an attribute referring to itself,
-through multiple attributes referring to each other,
-or through a combination of other [=arbitrary substitution functions=]
-(e.g. ''var()'').
-
-For each element, add a node for every attribute on that element
-to the graph described in [[css-variables-1#cycles]].
-Then, for each attribute <var>attrib</var>,
-add edges as follows:
-
-* From <var>attrib</var> to any attributes referenced by ''attr()''
-	within <var>attrib</var>'s value.
-* From <var>attrib</var> to any [=custom properties=] referenced by ''var()''
-	within <var>attrib</var>'s value.
-* From any [=custom property=] that references <var>attrib</var> using ''attr()''
-	to <var>attrib</var>.
-
-An ''attr()'' referencing an attribute
-which is part of a cycle
-makes the containing [=declaration=] [=invalid at computed-value time=].
-
-<div class='example'>
-	In the following,
-	<code>width</code> is [=invalid at computed-value time=],
-	because ''attr()'' tries to substitute a value which refers to itself:
-
-	<pre class='lang-css'>
-	div {
-	  width: attr(data-foo type(*));
-	}
-	</pre>
-	<pre class='lang-html'>
-	&lt;div data-foo="attr(data-foo type(*))">&lt;/div>
-	</pre>
-
-	Note: Even if one (or both) of the ''attr()'' functions had a fallback,
-	the result would be the same.
-</div>
-
-<div class='example'>
-	Cycles can occur through [=custom properties=];
-	in the following,
-	<code>--x</code> and <code>--y</code> are both [=invalid at computed-value time=]:
-
-	<pre class='lang-css'>
-	div {
-	  --x: var(--y);
-	  --y: attr(data-foo type(*));
-	}
-	</pre>
-	<pre class='lang-html'>
-	&lt;div data-foo="var(--x)">&lt;/div>
-	</pre>
-</div>
-
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