Remove the version number in README

It's cumbersome to have to edit it at release time, it can't be made a
README.in for display reasons (Github won't show it), and having the
version number here gives no special benefit.
Reviewed-by: NMatt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10205)

README

 
- OpenSSL 3.0.0-dev
-
  Copyright (c) 1998-2018 The OpenSSL Project
  Copyright (c) 1995-1998 Eric A. Young, Tim J. Hudson
  All rights reserved.