tests: Remove tst-zfs-simple.so

The tst-zfs-simple.so test case has serverd its purpose for bringup.  As
OSv build relies on working ZFS now, there's no need to run the tests.

Furthermore, we have the full ztest stress test in the tree:

  bsd/cddl/contrib/opensolaris/cmd/ztest/ztest.c

which we can use if needed.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>