remove utility that doesn't generate css

Having `decoration-[var(--abc)]` is ambiguous because there are multiple
plugins that have a `decoration` utility name. In order for this to work
you have to prefix it with the type: `decoration-[color:var(--abc)]`
which is already tested in this file.

Author: RobinMalfait

tests/arbitrary-values.test.css

@@ -822,9 +822,6 @@
 .decoration-\[color\:var\(--color\)\] {
   text-decoration-color: var(--color);
 }
-.decoration-\[var\(--color\)\] {
-  text-decoration-color: var(--color);
-}
 .decoration-\[length\:10px\] {
   text-decoration-thickness: 10px;
 }

tests/arbitrary-values.test.html

@@ -303,7 +303,6 @@
     <div class="decoration-[rgb(123,_123,_123)]"></div>
     <div class="decoration-[rgb(123_123_123)]"></div>
     <div class="decoration-[color:var(--color)]"></div>
-    <div class="decoration-[var(--color)]"></div>
 
     <div class="decoration-[length:10px]"></div>