MB-8322: Use couch_set_view for index deletion

Use the delete_index_dir/2 from couch_set_view to delete
indexes instead of a custom implementation.

For this change the corresponding couchdb change needs
to land first:
http://review.couchbase.org/26380

Change-Id: Ic013db5c18b05461e8d86d334bbca4a5c3c9b4d4
Reviewed-on: http://review.couchbase.org/26382
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>