Alter Table statement dropping default value not supported

### Dialect

MySQL

### Failing SQL

```sql
--Example: Products have a default stock of 0
ALTER TABLE products ALTER COLUMN stocks integer DROP DEFAULT;

--Example: Products are available by default (removed optional COLUMN keyword)
ALTER TABLE products ALTER available DROP DEFAULT;
```


### Description

https://dev.mysql.com/doc/refman/8.0/en/alter-table.html