MINOR; Remove duplicate updateLeaderEndOffsetAndTimestamp

[dengziming]

The KafkaRaftClient.onBecomeLeader will invoke appendLeaderChangeMessage, the call stack are:

1. appendLeaderChangeMessage
    1.1 flushLeaderLog
        1.1.1 updateLeaderEndOffsetAndTimestamp
        1.1.2 log.flush()

so the updateLeaderEndOffsetAndTimestamp is already invoked, and updateLeaderEndOffsetAndTimestamp should only be invoked after leo change(or time change), since log.flush() will not change leo(and time), it's unnessary to invoke twice.

[dengziming]

ping @hachikuji to have a look.