installation/executor.py: remove redundant Executor.submit() call

When the ThreadPoolExecutor is idle, this code:
```python
from concurrent.futures import wait, ThreadpoolExecutor
executor: ThreadpoolExecutor
wait([executor.submit(fn, arg)])
```
is equivalent to
```python
fn(arg)
```

Remove one such redundant call in installation/executor.py

src/poetry/installation/executor.py

@@ -168,7 +168,7 @@ def execute(self, operations: list[Operation]) -> int:
                 wait(tasks)
 
                 for operation in serial_operations:
-                    wait([self._executor.submit(self._execute_operation, operation)])
+                    self._execute_operation(operation)
 
             except KeyboardInterrupt:
                 self._shutdown = True