ECKey: remove mention of mailing list in documentation

.github/CONTRIBUTING.adoc

@@ -5,7 +5,6 @@ We love https://github.com/bitcoinj/bitcoinj/pulls[pull requests] from everyone!
 However, if you have a question or would like to start a discussion, please post to one of the following:
 
 * Our https://matrix.org[Matrix] room: https://matrix.to/#/#bitcoinj:matrix.org[#bitcoinj:matrix.org]
-* The https://groups.google.com/forum/#!forum/bitcoinj[bitcoinj google group]
 * https://bitcoin.stackexchange.com/questions/tagged/bitcoinj[Bitcoin stack exchange].
 
 We recommend opening an https://github.com/bitcoinj/bitcoinj/issue[GitHub Issue] or starting a discussion in https://matrix.to/#/#bitcoinj:matrix.org[#bitcoinj:matrix.org] _before_ creating a pull request with a significant change. This will allow other developers to give you feedback on your idea and how it might best align with **bitcoinj** and the development team's future plans for **bitcoinj**.

core/src/main/java/org/bitcoinj/crypto/ECKey.java

@@ -91,7 +91,7 @@
  *
  * <p>ECKey also provides access to Bitcoin Core compatible text message signing, as accessible via the UI or JSON-RPC.
  * This is slightly different to signing raw bytes - if you want to sign your own data and it won't be exposed as
- * text to people, you don't want to use this. If in doubt, ask on the mailing list.</p>
+ * text to people, you don't want to use this.</p>
  *
  * <p>The ECDSA algorithm supports <i>key recovery</i> in which a signature plus a couple of discriminator bits can
  * be reversed to find the public key used to calculate it. This can be convenient when you have a message and a