remove NotifyWatchonlyChanged

The signal is never called.

src/wallet/interfaces.cpp

@@ -535,10 +535,6 @@ public:
         return MakeSignalHandler(m_wallet->NotifyTransactionChanged.connect(
             [fn](const uint256& txid, ChangeType status) { fn(txid, status); }));
     }
-    std::unique_ptr<Handler> handleWatchOnlyChanged(WatchOnlyChangedFn fn) override
-    {
-        return MakeSignalHandler(m_wallet->NotifyWatchonlyChanged.connect(fn));
-    }
     std::unique_ptr<Handler> handleCanGetAddressesChanged(CanGetAddressesChangedFn fn) override
     {
         return MakeSignalHandler(m_wallet->NotifyCanGetAddressesChanged.connect(fn));