`allowed_index_name_length` was used for internal temporary operations
in SQLite3, since index name in SQLite3 must be globally unique and
SQLite3 doesn't have ALTER TABLE feature (so it is emulated by creating
temporary table with prefix).

`allowed_index_name_length` was to reserve the margin for the prefix,
but actually SQLite3 doesn't have a limitation for identifier name
length, so the margin has removed at 36901e6e.

Now `allowed_index_name_length` is no longer relied on by any adapter,
so I'd like to remove the internal specific method which is no longer
used.