Delete ForeignKey field fails (-607) column is referenced in index

[maxirobaina]

When django create a ForeignKey field, if db_index is not explicitly specified, It set by default db_index = True.
This behavior make that django migration module creates an extra index on Foreign Key field. In Firebird adding a foreign key constraints implicitly implies that an index is added.
Then, if delete a foreign key field is required, this operation will fail because Firebird want to delete the related index.

Engine Message :
unsuccessful metadata update
column [COLUMN_NAME] from table [TABLE_NAME is referenced in index [INDEX_NAME]

MySQL has a similar behavior.
https://github.com/django/django/blob/stable/1.8.x/django/db/backends/mysql/schema.py#L54