bsdinstall: Drop outdated comments about ZFS dataset compression

All datasets on zroot pool are compressed by default since FreeBSD 11.0 [1],
no need to mention that on some specific datasets these days.

[1] 47206692f2cca020891d1eec5028e02c3e6f56c5

Reviewed by: jrm (mentor), ziaee
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D52304