Remove unused code related with config option sqlite_db
Remove unused code related with config option sqlite_db

Config option sqlite_db is deprecated in
0a1bae9859079fb21a03716be947c5f1da6db0a2, and deprecate argument
sqlite_db in method set_defaults in
https://review.openstack.org/#/c/350945/, should use config option
connection instead. we test database with sqlite in memory
mode [1] as below:
CONF.set_default('connection', "sqlite://", group='database')

and don't use config option sqlite_db, so remove unused code.

[1]http://docs.sqlalchemy.org/en/rel_1_0/core/engines.html#sqlite

Change-Id: I32b1dcb367adac2323dce02a32dd063e0affa2b1

openstack/smaug,openstack/smaug