Cleanup libzpool/kernel.c

Cleanup libzpool/kernel.c

Commit
  ec213971274a ("async zvol minor node creation interferes with receive")
replaced zvol_create_minors with zvol_create_minor and
zvol_create_minors_recursive, changing the prototype at the same time.

However the stub functions in libzpool/kernel.c were defined with the
old prototype. As the definitions are empty, this doesn't cause any
runtime issues, but an LTO build shows warnings because of the
mismatched prototypes.

Commit
  a0bd735adb1b ("Add support for asynchronous zvol minor operations")
removed the real zvol_remove_minor, but for some reason added a stub
implementation in libzpool/kernel.c with no references. Delete this dead
code.

Commit
  196bee4cfd57 ("Remove deduplicated send/receive code")
removed zfs_onexit_del_cb and zfs_onexit_cb_data. Drop the stubs as
well.

Add zvol.h include to provide prototypes, and sort the include
directives.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Closes #10470