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fix statement leaking in executeQuery(String sql)

Running a river for hours leaks statementes if no timeout has been set.
The executeQuery(String sql) method doesn't close statement, neither
does the fetch() method close the statement associated with the
ResultSet.

Signed-off-by: Miso Korkiakoski <miso.korkiakoski@reaktor.fi>
remove unused method

Signed-off-by: Miso Korkiakoski <miso.korkiakoski@reaktor.fi>