Remove the redundant code

In methods 'version_control' and 'db_sync' in glance/db/sqlalchemy/migration.py,
there are redundant code 'sql_connection = CONF.sql_connection'.
The 'sql_connection's are local variable and not used.

Change-Id: Ie8a12e6f385f7103463172ed9acde1de0336a6d4
Closes-Bug:#1264056
Merge "Remove the redundant code"