doc: consolidate & remove release-note fragments

These have been moved here:
https://github.com/bitcoin-core/bitcoin-devwiki/wiki/24.0-Release-Notes-draft.

doc/release-notes/release-notes-24408.md

@@ -1,5 +0,0 @@
-New RPCs
---------
-
-- A new `gettxspendingprevout` RPC has been added, which scans the mempool to find
-  transactions spending any of the given outpoints. (#24408)

doc/release-notes/release-notes-471.md

@@ -1,4 +0,0 @@
-GUI changes
---------
-
-- A new menu item to restore a wallet from a backup file has been added (#471).