### Apache Airflow version

2.4.2

### What happened

Hi all,

I would like to update the scheduler_job.py script by removing the unused attribute -> `self.using_mysql`.

I understand that `self.using_sqlite` attribute is used to define the variable `async_mode` in scheduler_job.py code, but noticed that `self.using_mysql` is not used.

You can check the code from [scheduler_job.py](https://github.com/apache/airflow/blob/main/airflow/jobs/scheduler_job.py),
```python
def __init__(...):
    # Check what SQL backend we use
    sql_conn: str = conf.get_mandatory_value('database', 'sql_alchemy_conn').lower()
    self.using_sqlite = sql_conn.startswith('sqlite')   # <- only self.using_sqlite is used for async_mode
    self.using_mysql = sql_conn.startswith('mysql')

...

def _execute(self) -> None:
    ...
    async_mode = not self.using_sqlite
```

not sure it was used before, but no longer used for the current scheduler.

If it's not the major change, would you mind me to remove it from the script?

Thanks in advance!

### What you think should happen instead

_No response_

### How to reproduce

_No response_

### Operating System

-

### Versions of Apache Airflow Providers

_No response_

### Deployment

Other

### Deployment details

_No response_

### Anything else

_No response_

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

)
        
        
        pools = Pool.slots_stats(lock_rows=True, session=session)
        
        
        pool_slots_free = sum(max(0, pool['open']) for pool in pools.values())
        if pool_slots_free == 0:
            self.log.debug("All pools are full!")
            return []
        max_tis = min(max_tis, pool_slots_free)
        starved_pools = {pool_name for pool_name, stats in pools.items() if stats['open'] <= 0}
        
        dag_active_tasks_map: DefaultDict[str, int]
        task_concurrency_map: DefaultDict[tuple[str, str], int]
        dag_active_tasks_map, task_concurrency_map = self.__get_concurrency_maps(
            states=list(EXECUTION_STATES), session=session
        )
        num_tasks_in_executor = 0
        
        num_starving_tasks_total = 0
        
        starved_dags: set[str] = set()
        starved_tasks: set[tuple[str, str]] = set()
        pool_num_starving_tasks: DefaultDict[str, int] = defaultdict(int)
        for loop_count in itertools.count(start=1):
            num_starved_pools = len(starved_pools)
            num_starved_dags = len(starved_dags)
            num_starved_tasks = len(starved_tasks)