doc/go1.19: delete boringcrypto TODO

Boringcrypto has never been officially supported and it remains unsupported.
It need not be mentioned in the release notes.

Change-Id: I24a08d424982615244d51c1d250035d85a602023
Reviewed-on: https://go-review.googlesource.com/c/go/+/410362
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>

Author: rsc

doc/go1.19.html

@@ -196,10 +196,6 @@ <h2 id="linker">Linker</h2>
 
 <h2 id="library">Core library</h2>
 
-<p>
-  TODO: <a href="https://go.dev/issue/51940">https://go.dev/issue/51940</a>: all: move dev.boringcrypto into main branch behind GOEXPERIMENT
-</p>
-
 <p>
   TODO: complete this section
 </p>