Simplify truncate_query_log() callers

Simplify truncate_query_log() callers

truncate_query_log() returns NULL when statement truncation is
disabled or the supplied query does not need to be truncated. Therefore,
callers do not need to check log_statement_max_length before calling
it.

Remove the redundant checks and let truncate_query_log() make the
decision in one place. This simplifies the statement and duration
logging code without changing its behavior.

Author: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwFOV+7nOdfoO=kfVH=-fRA9aQE1YcHHLYty3nfQ9rQ4RA@mail.gmail.com