qt: drop unused watch-only functionality

The watch-only functionality in the GUI was only used for legacy wallets.
Watch-only descriptor wallets do not use this.

The only visible changes of this commit are:
- dropped "Spendable:" label from the overview tab
- column width cache is reset

src/qt/walletmodel.cpp

@@ -601,13 +601,7 @@ CAmount WalletModel::getAvailableBalance(const CCoinControl* control)
     // No selected coins, return the cached balance
     if (!control || !control->HasSelected()) {
         const interfaces::WalletBalances& balances = getCachedBalance();
-        CAmount available_balance = balances.balance;
-        // if wallet private keys are disabled, this is a watch-only wallet
-        // so, let's include the watch-only balance.
-        if (balances.have_watch_only && m_wallet->privateKeysDisabled()) {
-            available_balance += balances.watch_only_balance;
-        }
-        return available_balance;
+        return balances.balance;
     }
     // Fetch balance from the wallet, taking into account the selected coins
     return wallet().getAvailableBalance(*control);