Setting db_index=False on a ForeignKey doesn't actually remove the index

[Craig de Stigter]

1. Make a ForeignKey. Don't specify a value for db_index (django sets it to True by default)
2. Make a migration for the foreignkey. This creates an index:

CREATE INDEX "a_c_b_id_5a026661" ON "a_c" ("b_id");

3. Explicitly set db_index=False and create another migration
4. Check sqlmigrate for the new migration. Index is never actually removed:

BEGIN;
--
-- Alter field b on c
--
COMMIT;

One note: This is with postgres settings. I'm not sure if it's db-specific