doc/go1.12: remove known issue note

A workaround has been submitted.

Updates #27993

Change-Id: Ife6443c32673b38000b90dd2efb2985db37ab773
Reviewed-on: https://go-review.googlesource.com/c/156318Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.12.html

@@ -31,12 +31,6 @@ Do not send CLs removing the interior tags from such phrases.
   We expect almost all Go programs to continue to compile and run as before.
 </p>
 
-<p>
-  There is a <a href="https://golang.org/issue/27993"><strong>known issue</strong> in
-  the garbage collector</a> that can cause rare crashes. It is being investigated.
-  Please <a href="https://golang.org/issue/new">report</a> any issues you encounter.
-</p>
-
 <h2 id="language">Changes to the language</h2>
 
 <p>