Do not delete downloaded tracks on log out.

Downloaded tracks can be quite precious when you have lots of them (as
redownloading all of them can be costly in terms of time and money).
This prevents downloaded tracks from being deleted along with your
session.

This must be included in 0.21, so we can safely implement Funkwhale's
new authentication mechanism in 0.22.

app/src/main/java/com/github/apognu/otter/Otter.kt

@@ -87,8 +87,6 @@ class Otter : Application() {
     }
 
     cacheDir.resolve("picasso-cache").deleteRecursively()
-
-    exoDownloadManager.removeAllDownloads()
   }
 
   inner class CrashReportHandler : Thread.UncaughtExceptionHandler {