title: Cleanup flatpak repo folder?
body: <p>After trying to build the <a href="https://gitlab.gnome.org/GNOME/gitg" rel="noreferrer">gitg</a> flatpak I noticed my <code>/var/lib/flatpak/repo</code> folder has become very large. 
  <a href="https://i.stack.imgur.com/EPWlx.png" rel="noreferrer"><img src="https://i.stack.imgur.com/EPWlx.png" alt="enter image description here"></a>
  I'm assuming these are build files? Is there a good way to clean these up?</p>
  
  <p>I'm using Flatpak 1.4.0.</p>