Replace `struct update_lock_points` with lambda

No behavior change.
This code was introduced in 5add7a7 before we required C++11, which is
why the struct was needed. As we are now using more modern C++ and this
is the only place where lockpoints are updated for mempool entries, it
is more idiomatic to call `modify` with a lambda.

Co-authored-by: Hennadii Stepanov <32963518+hebasto@users.noreply.github.com>

src/txmempool.h

@@ -312,16 +312,6 @@ public:
     }
 };
 
-struct update_lock_points
-{
-    explicit update_lock_points(const LockPoints& _lp) : lp(_lp) { }
-
-    void operator() (CTxMemPoolEntry &e) { e.UpdateLockPoints(lp); }
-
-private:
-    const LockPoints& lp;
-};
-
 // Multi_index tag names
 struct descendant_score {};
 struct entry_time {};

src/validation.cpp

@@ -375,7 +375,7 @@ void CChainState::MaybeUpdateMempoolForReorg(
         } else if (!validLP) {
             // If CheckSequenceLocks succeeded, it also updated the LockPoints.
             // Now update the mempool entry lockpoints as well.
-            m_mempool->mapTx.modify(it, update_lock_points(lp));
+            m_mempool->mapTx.modify(it, [&lp](CTxMemPoolEntry& e) { e.UpdateLockPoints(lp); });
         }
 
         // If the transaction spends any coinbase outputs, it must be mature.