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 https://github.com/bitcoin/bitcoin/commit/e8661aac752eb08fee318eb8f56e599578d78f9f.
  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

doc/release-notes-32459.md

@@ -0,0 +1,3 @@
+GUI changes
+
+- Custom column widths in the Transactions tab are reset as a side-effect of legacy wallet removal. (#32459)