Update wildfly-elytron-tool/README to indicate that this repo is no longer used

wildfly-elytron-tool used to be a separate repository that contained the code for the Elytron Tool.
Now that all that code is in the wildfly-elytron repository in the tool folder (wildfly-elytron/tool), that repository is no longer used anymore.

The README for that repo needs to be updated to indicate that the wildfly-elytron-tool repo is not used and instead should look at wildfly-elytron/tool instead.

https://github.com/wildfly-security/wildfly-elytron-tool