Remove bogus taskq implementation from zpool.so

We had two implementation of the taskq_*() functions. One compiled into
the kernel (from bsd/sys/cddl/compat/opensolaris/kern/opensolaris_taskq.c)
and a second one compiled into libzfs
(bsd/cddl/contrib/opensolaris/lib/libzpool/common/taskq.c). The latter
is broken (e.g., uses the umem_zalloc() function we didn't even compile)
but was never called. But this causes bug #433, where we load libzfs.so
and forget to unload it (because of another bug), and then
tst-solaris-taskq fails because it uses the wrong taskq implementation.

So this patch drops the second, broken, taskq.c.

Fixes #433.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>