qe: fix a compiler warning when driver adapters are disabled (#4727)
Fixes the following warning when compiling the tests:
```
warning: unused variable: `adapter`
--> query-engine/request-handlers/src/load_executor.rs:28:29
|
28 | ConnectorKind::Js { adapter, _phantom } => {
| ^^^^^^^ help: try ignoring the field: `adapter: _`
|
= note: `#[warn(unused_variables)]` on by default
```