Merge bitcoin/bitcoin#32459: qt: drop unused watch-only functionality

e8661aac75 wallet: drop watch-only things from interface (Sjors Provoost)
e99188e7da qt: drop unused watch-only functionality (Sjors Provoost)

Pull request description:

  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 PR should be:
  - dropped "Spendable:" label from the overview tab
  - column width cache is reset

  This PR also removes some unused variables from the interface.

ACKs for top commit:
  davidgumberg:
    Review ACK e8661aac75.
  hebasto:
    ACK e8661aac75, I have reviewed the code and it looks OK. The `src/qt/forms/overviewpage.ui` form was reviewed in Qt Designer.

Tree-SHA512: d7edb0f167e0b934075398a76eddca69890bb36848a918c932b1c2cea85ee87285e876cbfdf1f6dec7adf26b9f405fb558c70bec0c84585c0a9df33c2af78393

src/wallet/interfaces.cpp

@@ -399,7 +399,6 @@ public:
         result.balance = bal.m_mine_trusted;
         result.unconfirmed_balance = bal.m_mine_untrusted_pending;
         result.immature_balance = bal.m_mine_immature;
-        result.have_watch_only = false;
         return result;
     }
     bool tryGetBalances(WalletBalances& balances, uint256& block_hash) override