spec: remove sentence discussing existing implementations
spec: remove sentence discussing existing implementations

Fixes #22282.

Change-Id: I4097e9cbd590ab6c8b1511a3b752c6ac98ac819b
Reviewed-on: https://go-review.googlesource.com/72792
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>

cloudflare/go,Cofyc/go,golang/go,nawawi/go,golang/go,golang/go,akutz/go,nawawi/go,nawawi/go,cloudflare/go,CAFxX/go,CAFxX/go,Cofyc/go,codestation/go,codestation/go,golang/go,josharian/go,codestation/go,akutz/go,codestation/go,nawawi/go,CAFxX/go,akutz/go,codestation/go,akutz/go,akutz/go,josharian/go,golang/go,CAFxX/go,josharian/go,Cofyc/go,cloudflare/go,codestation/go,Cofyc/go,akutz/go,Cofyc/go,cloudflare/go,CAFxX/go,josharian/go,akutz/go,golang/go,CAFxX/go,cloudflare/go,josharian/go,codestation/go,cloudflare/go,codestation/go,CAFxX/go,nawawi/go,nawawi/go,golang/go,Cofyc/go,josharian/go,akutz/go,cloudflare/go,CAFxX/go,josharian/go,josharian/go,cloudflare/go,nawawi/go,Cofyc/go,christopher-henderson/Go,Cofyc/go,golang/go,nawawi/go