events: remove unreachable code

Commit 8d386ed7 stopped the
Event Emitter implementation from storing arrays containing a
single listener. This change left a section of code in
removeListener() as unreachable. This commit removes the
unreachable code.

Refs: https://github.com/nodejs/node/pull/12043
PR-URL: https://github.com/nodejs/node/pull/12501


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>