Also delete local messages when using "clear messages" on an account

We have been throwing away all attachments already, so it doesn't make
too much sense to keep local messages. And when we're not keeping local
messages we can remove all entries from the 'threads' table.

k9mail/src/main/java/com/fsck/k9/mailstore/LocalStore.java

@@ -285,24 +285,16 @@ public class LocalStore extends Store implements Serializable {
 
             Log.i(K9.LOG_TAG, "After prune / before clear size = " + getSize());
         }
-        // don't delete messages that are Local, since there is no copy on the server.
-        // Don't delete deleted messages.  They are essentially placeholders for UIDs of messages that have
-        // been deleted locally.  They take up insignificant space
+
         database.execute(false, new DbCallback<Void>() {
             @Override
             public Void doDbWork(final SQLiteDatabase db) {
-                // Delete entries from 'threads' table
-                db.execSQL("DELETE FROM threads WHERE message_id IN " +
-                        "(SELECT id FROM messages WHERE deleted = 0 AND uid NOT LIKE 'Local%')");
+                // We don't care about threads of deleted messages, so delete the whole table.
+                db.delete("threads", null, null);
 
-                // Set 'root' and 'parent' of remaining entries in 'thread' table to 'NULL' to make
-                // sure the thread structure is in a valid state (this may destroy existing valid
-                // thread trees, but is much faster than adjusting the tree by removing messages
-                // one by one).
-                db.execSQL("UPDATE threads SET root=id, parent=NULL");
-
-                // Delete entries from 'messages' table
-                db.execSQL("DELETE FROM messages WHERE deleted = 0 AND uid NOT LIKE 'Local%'");
+                // Don't delete deleted messages. They are essentially placeholders for UIDs of messages that have
+                // been deleted locally.
+                db.delete("messages", "deleted = 0", null);
                 return null;
             }
         });