Delete unused zfsctl_snapdir_inactive declaration

zfsctl_snapdir_inactive is defined in zfs-0.6.3. In zfs-0.6.5.7
this is declaration remains even though the implementation was
removed in commit 278bee93. Removed fastreboot_disable_highpil
which is also unused.

Signed-off-by: caoxuewen cao.xuewen@zte.com.cn
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #5042