Before this patch, _sequence_data system space wasn't taken into
consideration when space was dropped. However, entries in this space
may appear after recovery. For example:

CREATE TABLE t (id INT PRIMARY KEY AUTOINCREMENT);
INSERT INTO t VALUES (NULL);
box.snapshot()
os.exit()
...
box.cfg{}
DROP TABLE t;

Last DROP statement didn't generate VDBE code to delete entry from
_sequence_data, but it had to do so. This patch simply modifies code
generation to remove entry from _sequence_data space before space is
dropped.

Closes #3712