doc/go1.18: remove ppc regabi TODO as it's in the compiler section

Updates #47694

Change-Id: I55175988f193fb573339933e9ff0d4c49734b444
Reviewed-on: https://go-review.googlesource.com/c/go/+/385658
Trust: Jeremy Faller <jeremy@golang.org>
Run-TryBot: Jeremy Faller <jeremy@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>

doc/go1.18.html

@@ -225,12 +225,6 @@ Do not send CLs removing the interior tags from such phrases.
   FreeBSD 13.0+ will require a kernel with the COMPAT_FREEBSD12 option set (this is the default).
 </p>
 
-<h3 id="ppc64">PPC64</h3>
-
-<p><!-- CL 353969 -->
-  TODO: <a href="https://golang.org/cl/353969">https://golang.org/cl/353969</a>: enable register ABI for PPC64
-</p>
-
 <h2 id="tools">Tools</h2>
 
 <h3 id="fuzzing">Fuzzing</h3>