database: remove unused concurrency config members
database: remove unused concurrency config members

Signed-off-by: Botond Dénes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <b257c7e9d403c55aaec34fc48863c18f9c9ae11a.1517314398.git.a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>

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