[DBAgentSQLite3] Remove an initialization mechanism.

Now initialization of tables is done in DBTables and the sub
classes. So the complicated mechanism in DBAgentSQLite3 is no longer
used.

I'll also remove the initialization mechanism in DBAgentMySQL
[DBAgentTest][testDBAgentSQLite3] Remove a global variable to improve flexibility..
Merge pull request #488 from project-hatohol/cleanup-dbagent

Cleanup dbagent