Use default sshd_config from bookworm.

There is no need to override the settings after all, as the
sshd_config.d settings take precendence, see:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1014234#10

Verified that this is true with "sudo sshd -T".

It is a bit counter-intuitive, but it works!