refactor: inline constant return value of CDBWrapper::WriteBatch

`WriteBatch` can only ever return `true` - its errors are handled by throwing a `throw dbwrapper_error` instead.
The boolean return value is quite confusing, especially since it's symmetric with `CDBWrapper::Read`, which catches the exceptions and returns a boolean instead.
We're removing the constant return value and inlining `true` for its usages.

src/dbwrapper.h

@@ -234,7 +234,8 @@ public:
     {
         CDBBatch batch(*this);
         batch.Write(key, value);
-        return WriteBatch(batch, fSync);
+        WriteBatch(batch, fSync);
+        return true;
     }
 
     //! @returns filesystem path to the on-disk data.
@@ -259,10 +260,11 @@ public:
     {
         CDBBatch batch(*this);
         batch.Erase(key);
-        return WriteBatch(batch, fSync);
+        WriteBatch(batch, fSync);
+        return true;
     }
 
-    bool WriteBatch(CDBBatch& batch, bool fSync = false);
+    void WriteBatch(CDBBatch& batch, bool fSync = false);
 
     // Get an estimate of LevelDB memory usage (in bytes).
     size_t DynamicMemoryUsage() const;