Merge bitcoin/bitcoin#22772: refactor: hasher cleanup (follow-up to 19935)

4c69571e6e doc: remove outdated comment (Martin Zumsande)
16652a93ea refactor: Remove unused KeyIDHasher (Martin Zumsande)

Pull request description:

  Small follow-ups to  #19935:

  - Removal of unused `KeyIDHasher`  class ([comment in 19935](https://github.com/bitcoin/bitcoin/pull/19935#discussion_r544464524))
  - Removal of an outdated comment, which referred to an old problem with the no longer supported Boost 1.46 and `boost::unordered_map`, now replaced by `std::unordered_map`. ([comment in 19935](https://github.com/bitcoin/bitcoin/pull/19935#discussion_r540911134))

ACKs for top commit:
  Saviour1001:
    Tested ACK <code>[4c69571](4c69571e6e)</code>
  Zero-1729:
    ACK 4c69571e6e
  theStack:
    ACK 4c69571e6e 🆗

Tree-SHA512: 243fda2120bfac6c40a268ca2c0f34482ce27e71fbc50005c0d13c2ad5db9ee72a037f9937c37cc50ed0f9f6f11ee6afee4ac50e5031d6876ec942f41f38dadf

src/util/hasher.h

@@ -33,10 +33,6 @@ public:
     SaltedOutpointHasher();
 
     /**
-     * This *must* return size_t. With Boost 1.46 on 32-bit systems the
-     * unordered_map will behave unpredictably if the custom hasher returns a
-     * uint64_t, resulting in failures when syncing the chain (#4634).
-     *
      * Having the hash noexcept allows libstdc++'s unordered_map to recalculate
      * the hash during rehash, so it does not have to cache the value. This
      * reduces node's memory by sizeof(size_t). The required recalculation has

src/wallet/scriptpubkeyman.h

@@ -148,17 +148,6 @@ public:
     }
 };
 
-class KeyIDHasher
-{
-public:
-    KeyIDHasher() {}
-
-    size_t operator()(const CKeyID& id) const
-    {
-        return id.GetUint64(0);
-    }
-};
-
 /*
  * A class implementing ScriptPubKeyMan manages some (or all) scriptPubKeys used in a wallet.
  * It contains the scripts and keys related to the scriptPubKeys it manages.