daemon: reset CURRENT_WALLET if the wallet gets deleted

Set SimpleConfig.CURRENT_WALLET = None if the wallet gets deleted,
otherwise we try to open it on the next startup which will show an error
message in QML.

electrum/daemon.py

@@ -562,6 +562,8 @@ class Daemon(Logger):
         if os.path.exists(path):
             os.unlink(path)
             self.update_recently_opened_wallets(path, remove=True)
+            if self.config.CURRENT_WALLET == path:
+                self.config.CURRENT_WALLET = None
             return True
         return False