clean up db when deleting a repository

Previously everything from a repo staying inside the db when removing it
without disabling it first, the problem manifests when the repo is
readded later (or a mirror), as it would get a new id but all apk
entries still point to the original repoid.

So we now first disable a repo (which just calls
RepoProvider.Helper.purgeApps) before deleting it from the db.

closes Bubu/fdroidclassic#29

app/src/main/java/org/fdroid/fdroid/data/RepoProvider.java

@@ -233,6 +233,7 @@ public class RepoProvider extends FDroidProvider {
         }
 
         public static void remove(Context context, long repoId) {
+            purgeApps(context, findById(context, repoId));
             ContentResolver resolver = context.getContentResolver();
             Uri uri = RepoProvider.getContentUri(repoId);
             resolver.delete(uri, null, null);