dmu.h: remove stale declaration dmu_objset_snapshot_tmp

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Adam Moss <c@yotes.com>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes openzfs#11047

include/sys/dmu.h

@@ -337,7 +337,6 @@ int dmu_objset_clone(const char *name, const char *origin);
 int dsl_destroy_snapshots_nvl(struct nvlist *snaps, boolean_t defer,
     struct nvlist *errlist);
 int dmu_objset_snapshot_one(const char *fsname, const char *snapname);
-int dmu_objset_snapshot_tmp(const char *, const char *, int);
 int dmu_objset_find(const char *name, int func(const char *, void *), void *arg,
     int flags);
 void dmu_objset_byteswap(void *buf, size_t size);