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

This reverts commit 1728b8b.

There's an existing spec for this:

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

Author: brixen

optional/capi/enumerator_spec.rb

@@ -25,15 +25,6 @@
           enumerator = @s.rb_enumeratorize(@enumerable, :each, :arg1, :arg2)
           enumerator.class.should == Enumerator
         end
-        
-        it 'raises an exception when the enumerator position is already at the end' do
-          e = 2.times
-          e.peek.should == 0
-          e.next
-          e.peek.should == 1
-          e.next
-          lambda { e.peek }.should raise_error(StopIteration)
-        end
       end
 
       it "enumerates the given object" do