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

The duplication originated in cdd46c76, 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.