Switch to Reline backend for readline

[eregon]

Following the logic of https://github.com/ruby/readline/blob/master/lib/readline.rb We have that file at https://github.com/oracle/truffleruby/blob/master/lib/mri/readline.rb But we also have https://github.com/oracle/truffleruby/blob/master/lib/truffle/readline.rb and that's earlier in $LOAD_PATH.

So we should try to just remove the JLine backend for readline entirely (and we don't have to maintain it anymore) and it should just work by using Reline.

Related: https://bugs.ruby-lang.org/issues/19616