tests/kernel/mem_pool/mem_pool_api: Fix stack sizes

The (artificially small) ISR stack was overflowing on this test when
CONFIG_DEBUG was enabled on qemu_x86.  Really there's no reason to be
restricting stack size at all in a memory pool test, just remove those
settings and use the defaults, which are fine.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>