Revert "Fix on_eof behavior when the consumer group is rebalanced"

This reverts commit e625c64c as the consumers are now lost in an infinite loop after a partition rebalance.

This commit simply revert the change because the on_eof option is almost not yet used by any consumer in production (only the journal scrubber). The proper way to implemennt the change will be investigate later.

Related to #4659 (closed)