Remove silent_mode. You get the same functionality with "pg_ctl -l

Remove silent_mode. You get the same functionality with "pg_ctl -l
postmaster.log", or nohup.

There was a small issue with LINUX_OOM_ADJ and silent_mode, namely that with
silent_mode the postmaster process incorrectly used the OOM settings meant
for backend processes. We certainly could've fixed that directly, but since
silent_mode was redundant anyway, we might as well just remove it.