README: remove stale info about upcoming edition change

It is done.

Author: dpc

README.md

@@ -18,12 +18,6 @@
   </p>
 </div>
 
-**Heads up for contributors: upcoming edition change**
-
-We're currently preparing to bump MSRV and **change the edition to 2018**.
-To minimize the churn we recommend you to submit your local WIP changes ASAP.
-There will be a lot of rebasing after the edition change.
-
 [Documentation](https://docs.rs/bitcoin/)
 
 Supports (or should support)