Hides tabline type as the doc says

From the documentation:
```
* enable/disable displaying tab type (far right)
  let g:airline#extensions#tabline#show_tab_type = 1
```
Which suggests to me that it only makes sense to hide the tab type for both buffer and tab types. This PR fixes that.

@bling thanks for the amazing plugin.
I could be wrong -- I'm not really familiar with this code, but shouldn't this be "buffers", not "tabs"? This line is inside the ```s:get_buffers()``` function, which I think means the tabline is showing buffers.