Remove unneeded "extern inline" function declarations

All of these externs are already #included as static inline
functions via corresponding headers.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Closes #13073

module/zfs/dsl_dir.c

@@ -121,8 +121,6 @@
  * dsl_dir_init_fs_ss_count().
  */
 
-extern inline dsl_dir_phys_t *dsl_dir_phys(dsl_dir_t *dd);
-
 static uint64_t dsl_dir_space_towrite(dsl_dir_t *dd);
 
 typedef struct ddulrt_arg {