wallet: remove unused WalletDatabase::m_refcount

The counter is never incremented. Only the BDB implementation ever
maintained it, and that went away in 04a7a7a28c ("build, wallet, doc:
Remove BDB"). `AddRef()` and `RemoveRef()`, which the comment above the
member describes as maintaining it, were removed in c0f3f3264f
("wallet: Remove unused db functions"), leaving the member behind.

src/wallet/db.h

@@ -11,7 +11,6 @@
 #include <support/allocators/secure.h>
 #include <util/fs.h>
 
-#include <atomic>
 #include <memory>
 #include <optional>
 #include <string>
@@ -136,9 +135,6 @@ public:
     /** Open the database if it is not already opened. */
     virtual void Open() = 0;
 
-    //! Counts the number of active database users to be sure that the database is not closed while someone is using it
-    std::atomic<int> m_refcount{0};
-
     /** Rewrite the entire database on disk
      */
     virtual bool Rewrite() = 0;