say: get rid of syslog formatter

All messages written to syslog must have a header, which contains the
current time, message severity, and process identity, so we have a
special formatter function for syslog. As a result, the syslog log
destination is incompatible with the json log format, which has its own
formatter function. This is confusing, because there's nothing that
prevents us from writing json in the log entry body to syslog - we just
need to prepend the message with a proper header.

As a preparation for befriending json and syslog, let's get rid of the
syslog formatter function and instead write the header right in log_vsay
in case logs are written to syslog. We just need to strip the duplicate
information from the log entry in say_format_plain so as not to print
the time and pid twice.

Needed for #7860

NO_DOC=refactoring
NO_TEST=refactoring
NO_CHANGELOG=refactoring