Don't duplicate log_checkpoint messages for both of restart and checkpoints.

 Don't duplicate log_checkpoint messages for both of restart and checkpoints.
 
 The duplication originated in cdd46c765, where restartpoints were
 introduced.
 
 In LogCheckpointStart's case the duplication actually lead to the
 compiler's format string checking not to be effective because the
 format string wasn't constant.
 
 Arguably these messages shouldn't be elog(), but ereport() style
 messages. That'd even allow to translate the messages... But as
 there's more mistakes of that kind in surrounding code, it seems
 better to change that separately.