kernel: remove duplicate definitions of k_thread stack and entry

Both k_thread_stack_t and (*k_thread_entry_t) are defined in
include/kernel.h and include/sys/arch_interface.h. The latter is
indirectly included by kernel.h which causes issues with some
toolchains. So remove the definitions in kernel.h.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>