tests/kernel/mem_protect: Remove mem_pool test case

The sys_mem_pool data structure is going away.  And this test case
didn't actually do much.  All it did was create a sys_mem_pool in the
app data section (I guess that's the "mem_protect" part?) and validate
that it was usable.  We have tests for sys_heap to do that already
elsewhere anyway; no point in porting.

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