refactor: inline constant return value of CDBWrapper::Erase and BlockTreeDB::WriteReindexing

Did both in this commit, since the return value of `WriteReindexing` was ignored anyway - which existed only because of the constant `Erase` being called

src/dbwrapper.h

@@ -255,12 +255,11 @@ public:
     }
 
     template <typename K>
-    bool Erase(const K& key, bool fSync = false)
+    void Erase(const K& key, bool fSync = false)
     {
         CDBBatch batch(*this);
         batch.Erase(key);
         WriteBatch(batch, fSync);
-        return true;
     }
 
     void WriteBatch(CDBBatch& batch, bool fSync = false);