Alter schema: allow to drop map fields

### Feature Request / Improvement

It happens from time to time: we used a map field in the table and we stopped to use it and thus want to remove the field from the schema.
Currently this is not allowed with the following error:
```java.lang.IllegalArgumentException: Cannot delete map keys: map```

### Query engine

None

### Willingness to contribute

- [ ] I can contribute this improvement/feature independently
- [x] I would be willing to contribute this improvement/feature with guidance from the Iceberg community
- [ ] I cannot contribute this improvement/feature at this time