Remove duplicated key_pair* tests from test_db_api.

There is some key_pair* tests in old DbApiTestCase
which are duplicated in new KeyPairTestCase.
This patch simply remove them.

blueprint db-api-tests

Change-Id: Ie25054590286d994bc6748ca85944fa65dcf3d4d
Merge "Remove duplicated key_pair* tests from test_db_api."