flatpak-bisect: Remove unused variable

This fixes a warning pointed out by lgtm.com. I checked that the following
still works:
$ PAGER=more flatpak-bisect -b stable org.gnome.Builder log

Closes: #2894
Approved by: matthiasclasen

scripts/flatpak-bisect

@@ -180,7 +180,7 @@ class Bisector():
             stdout = None
         p = subprocess.Popen(cmd, stdout=stdout)
         if pager:
-            ps = subprocess.check_call((pager), stdin=p.stdout)
+            subprocess.check_call((pager), stdin=p.stdout)
         p.wait()
 
     def checkout(self, commit=None):