Remove code for zfs remap

The "zfs remap" command was disabled by
6e91a72fe3, because it has little utility
and introduced some tricky bugs.  This commit removes the code for it,
the associated ZFS_IOC_REMAP ioctl, and tests.

Note that the ioctl and property will remain, but have no functionality.
This allows older software to fail gracefully if it attempts to use
these, and avoids a backwards incompatibility that would be introduced if
we renumbered the later ioctls/props.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #8944

include/sys/dsl_dir.h

@@ -49,7 +49,6 @@ struct dsl_dataset;
 #define	DD_FIELD_FILESYSTEM_COUNT	"com.joyent:filesystem_count"
 #define	DD_FIELD_SNAPSHOT_COUNT		"com.joyent:snapshot_count"
 #define	DD_FIELD_CRYPTO_KEY_OBJ		"com.datto:crypto_key_obj"
-#define	DD_FIELD_LAST_REMAP_TXG		"com.delphix:last_remap_txg"
 
 typedef enum dd_used {
 	DD_USED_HEAD,
@@ -154,7 +153,6 @@ void dsl_dir_stats(dsl_dir_t *dd, nvlist_t *nv);
 uint64_t dsl_dir_space_available(dsl_dir_t *dd,
     dsl_dir_t *ancestor, int64_t delta, int ondiskonly);
 void dsl_dir_dirty(dsl_dir_t *dd, dmu_tx_t *tx);
-int dsl_dir_get_remaptxg(dsl_dir_t *dd, uint64_t *count);
 void dsl_dir_sync(dsl_dir_t *dd, dmu_tx_t *tx);
 int dsl_dir_tempreserve_space(dsl_dir_t *dd, uint64_t mem,
     uint64_t asize, boolean_t netfree, void **tr_cookiep, dmu_tx_t *tx);
@@ -172,7 +170,6 @@ int dsl_dir_activate_fs_ss_limit(const char *);
 int dsl_fs_ss_limit_check(dsl_dir_t *, uint64_t, zfs_prop_t, dsl_dir_t *,
     cred_t *);
 void dsl_fs_ss_count_adjust(dsl_dir_t *, int64_t, const char *, dmu_tx_t *);
-int dsl_dir_update_last_remap_txg(dsl_dir_t *, uint64_t);
 int dsl_dir_rename(const char *oldname, const char *newname);
 int dsl_dir_transfer_possible(dsl_dir_t *sdd, dsl_dir_t *tdd,
     uint64_t fs_cnt, uint64_t ss_cnt, uint64_t space, cred_t *);