readline: clean up event listener in onNewListener
If this is a bug, then let's just fix it.
Refs: https://github.com/nodejs/node/pull/9447#discussion_r86599214
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines
Affected core subsystem(s)
readline