say: enable json log format with syslog

All the heavy lifting was done in the previous commit that dropped
the syslog formatter. This commit just removes the checks that forbid
using syslog with json.

Closes #7860

@TarantoolBot document
Title: JSON log format can now be used with syslog

The configuration reference says that syslog is incompatible with
the JSON log format:

https://www.tarantool.io/en/doc/latest/reference/configuration/#confval-log_format

This isn't true anymore. If JSON is used with syslog, then a JSON
message body is appended to the syslog header.

See https://github.com/tarantool/tarantool/issues/7860.