removes text from back button text

fixes #2193 

cc @aerych 
I have mixed feelings about this since its essentially manipulating the title of another view controller. ReaderPostViewController resets its title in `viewWillAppear` which is why, after this patch, we're not seeing a blank title when returning to the post list 

How do you feel about using the same trick shown in lines 119 - 121 in the ReaderPostViewController instead? Seems like that approach has better encapsulation.