tmem: remove dumb check in do_tmem_destroy_pool

tmem: remove dumb check in do_tmem_destroy_pool

do_tmem_destroy_pool is checking if pools == NULL. But, pools is a fixed
array.

Clang 3.5 will fail to compile xen/common/tmem.c with the following error:
tmem.c:1848:18: error: comparison of array 'client->pools' equal to a null
pointer is always false [-Werror,-Wtautological-pointer-compare]
    if ( client->pools == NULL )

Coverity-ID:1055632

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>