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

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)