Remove references to the mailing lists

Since the git-crypt mailing lists have barely been used, and mailing
lists seem to be falling out of fashion for open source projects, I've
decided to shut down the git-crypt mailing lists in favor of functionality
provided by GitHub.

For announcements of new releases, you can watch the git-crypt
repository (https://github.com/AGWA/git-crypt) for new releases.

For bug reports, you can file an issue:
https://github.com/AGWA/git-crypt/issues

For discussions, you can use GitHub's new discussions feature:
https://github.com/AGWA/git-crypt/discussions

CONTRIBUTING.md

@@ -4,8 +4,7 @@ documentation, bug reports, or anything else that improves git-crypt.
 
 When contributing code, please consider the following guidelines:
 
- * You are encouraged to open an issue on GitHub or send mail to
-   git-crypt-discuss@lists.cloudmutt.com to discuss any non-trivial
+ * You are encouraged to open an issue on GitHub to discuss any non-trivial
    changes before you start coding.
 
  * Please mimic the existing code style as much as possible.  In
@@ -15,8 +14,7 @@ When contributing code, please consider the following guidelines:
  * To minimize merge commits, please rebase your changes before opening
    a pull request.
 
- * To submit your patch, open a pull request on GitHub or send a
-   properly-formatted patch to git-crypt-discuss@lists.cloudmutt.com.
+ * To submit your patch, open a pull request on GitHub.
 
 Finally, be aware that since git-crypt is security-sensitive software,
 the bar for contributions is higher than average.  Please don't be

README

@@ -158,12 +158,3 @@ match it accidentally.  If necessary, you can exclude .gitattributes from
 encryption like this:
 
 	.gitattributes !filter !diff
-
-
-MAILING LISTS
-
-To stay abreast of, and provide input to, git-crypt development, consider
-subscribing to one or both of our mailing lists:
-
-Announcements: https://lists.cloudmutt.com/mailman/listinfo/git-crypt-announce
-Discussion:    https://lists.cloudmutt.com/mailman/listinfo/git-crypt-discuss

README.md

@@ -160,12 +160,3 @@ match it accidentally.  If necessary, you can exclude .gitattributes from
 encryption like this:
 
     .gitattributes !filter !diff
-
-Mailing Lists
--------------
-
-To stay abreast of, and provide input to, git-crypt development,
-consider subscribing to one or both of our mailing lists:
-
-* [Announcements](https://lists.cloudmutt.com/mailman/listinfo/git-crypt-announce)
-* [Discussion](https://lists.cloudmutt.com/mailman/listinfo/git-crypt-discuss)