Revert "Added specs for Enumerator#peek behaviour. When the Enumerator position is already at the end and method peek is called."

This reverts commit 1728b8bcfd826e9105270d1ffa26a59724c250e0.

There's an existing spec for this:

https://github.com/rubyspec/rubyspec/blob/master/core/enumerator/peek_spec.rb#L33-36