Fix for discarding a post in non-visual editor.

Fixes: https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/176

/cc @diegoreymendez @sendhil 

@bummytime - I've been recently through something similar in the new editor, and solved it initially in a pretty similar way.

Then I suddenly realized that it could be cleaner if I moved the code from

`-(void)actionSheet:(UIActionSheet *)actionSheet clickedButtonAtIndex:(NSInteger)buttonIndex`

to

`-(void)actionSheet:(UIActionSheet *)actionSheet didDismissWithButtonIndex:(NSInteger)buttonIndex`.

From what I'm seeing, it may be the same case here.  By moving the code to execute once the action sheet goes away, you could completely eliminate the property `dismissingEditView`.