Revert "doc/go1.18: document type parameter name restriction"

This reverts CL 376414.

For golang#47694.
For golang#50481.

Change-Id: Ie73961046e52e6e5d3262ef0aeaa24bec7eaa937
Reviewed-on: https://go-review.googlesource.com/c/go/+/379835
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.18.html

@@ -108,12 +108,6 @@ <h3 id="generics">Generics</h3>
       interface type with a non-empty method set. Whether this will
       ever be permitted is unclear at present.
     </li>
-    <li><!-- https://golang.org/issue/50481 -->
-      A generic type or function currently may declare at most one blank (<code>_</code>)
-      type parameter name. Note that it is always possible to use an arbitrary new
-      (unused) identifier in place of a blank type parameter name.
-      We plan to remove this restriction in Go 1.19.
-    </li>
   </ul>
 </p>