Fix property name in code comment

The property name of `Ember.Array#hasArrayObservers` is incorrect.
Remove unexist param's comment

`Ember.Enumerable#enumerableContentDidChange` allow to be given two arguments.
* `removing`
* `adding`

Not contains `start`.
[beta] Remove unused variable in test
Merge pull request #3959 from pangratz/cleanup_test

Remove unused variable in test
Merge pull request #3957 from tricknotes/fix-code-comment

Fix code comment