newsyslog(8): Remove the -c command line option.

The -c command-line option, which allowed setting a global compression
method, is removed in favor of the <compress> directive in newsyslog.conf.

The motivation of this change is to eliminate a source of potential confusion.
Since newsyslog(8) is typically invoked by cron(8), having a command line
option that can override it could cause inconsistent behavior (specifying
in crontab would take precedence).

Suggested by: karels
Differential Revision: https://reviews.freebsd.org/D43466