Retire zfs_autoimport_disable kmod option

Back in 2014 the zfs_autoimport_disable module option was added to
control whether the kmods should load the pool configs from the cache
file on module load.  The default value since that time has been for
the kernel to not process the cache file.

Detecting and importing pools during boot is now controlled outside
of the kmod on both Linux and FreeBSD.  By all accounts this has been
working well and we can remove this dormant code on the kernel side.

The spa_config_load() function is has been moved to userspace, it is
now only used by libzpool.  Additionally, the spa_boot_init() hook
which was used by FreeBSD now looks to be used and was removed.

Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #17618

include/sys/spa.h

@@ -880,7 +880,6 @@ extern kcondvar_t spa_namespace_cv;
 #define	SPA_CONFIG_UPDATE_VDEVS	1
 
 extern void spa_write_cachefile(spa_t *, boolean_t, boolean_t, boolean_t);
-extern void spa_config_load(void);
 extern int spa_all_configs(uint64_t *generation, nvlist_t **pools);
 extern void spa_config_set(spa_t *spa, nvlist_t *config);
 extern nvlist_t *spa_config_generate(spa_t *spa, vdev_t *vd, uint64_t txg,
@@ -1244,7 +1243,6 @@ extern void vdev_mirror_stat_fini(void);
 /* Initialization and termination */
 extern void spa_init(spa_mode_t mode);
 extern void spa_fini(void);
-extern void spa_boot_init(void *);
 
 /* properties */
 extern int spa_prop_set(spa_t *spa, nvlist_t *nvp);