Enable setting 'write.format.default'

Before IMPALA-10610 we didn't support Iceberg tables with mixed-format data files.

And we used 'write.format.default' to determine the file format of the table, which was only a temporary workaround. Because of this we didn't allow changing this table property if the table already contained different table formats. E.g. we didn't allow modifying 'write.format.default' to PARQUET if the table already contained ORC files, because it would have made the table unreadable for Impala.

Since IMPALA-10610 'write.format.default' is not used to determine the Iceberg table's format anymore, so we can allow changing it.