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/test/dbwrapper_tests.cpp

@@ -173,7 +173,7 @@ BOOST_AUTO_TEST_CASE(dbwrapper_batch)
         // Remove key3 before it's even been written
         batch.Erase(key3);
 
-        BOOST_CHECK(dbw.WriteBatch(batch));
+        dbw.WriteBatch(batch);
 
         BOOST_CHECK(dbw.Read(key, res));
         BOOST_CHECK_EQUAL(res.ToString(), in.ToString());