Remove vestigial settings related to initramfs

Remove ZFS_POOL_IMPORT, ZFS_INITRD_PRE_MOUNTROOT_SLEEP,
ZFS_INITRD_POST_MODPROBE_SLEEP, and ZFS_INITRD_ADDITIONAL_DATASETS
features from etc/defaults/zfs.in. These features no longer work.

Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Chris McDonough <chrism@plope.com>
Closes #9126
Closes #10757