sshd_config: Do not set defaults explicitly

sshd_config: Do not set defaults explicitly

In order to keep configurations as small as possible and to make them
easier to read/audit, this patch omits all default configuration in the
OpenSSH server configuration file.

Further, it mentions where to refer for the full documentation.

Signed-off-by: Peter Müller <peter.mueller@ipfire.org>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>