[BUGFIX beta] Remove unreferenced function from ember-metal/replace

[tricknotes]

This function became unnecessary with the followings:

• Ember.ArrayController: 9899dff
• Ember.Select: 4b5b972

I think we can remove this function due to un-exported.
If are there some fears, should it be shown deprecation warning?

[rwjblue]

I wondered if we needed _replace also, but it looks like the _replace function is still being used in ember-runtime/system/native_array.

Maybe we should just drop the underscore, and make it the default export in this file?

[tricknotes]

Completely agreed 👍
I updated my commit.

[rwjblue]

Thanks again!