[dev.tls] dev.tls: remove sentinel file

Now that we're branched in the history we don't need this anymore.

Change-Id: I0655b2496e273aec87e77557bc4baa06bbc8cfcf
Reviewed-on: https://go-review.googlesource.com/33109


Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

dev.tls

-Branch for working on TLS 1.3