kernel: remove deprecated STACK EXTERN macros

These macros have been deprecated since v3.2.0. So remove them.

() K_KERNEL_STACK_EXTERN
   Use K_KERNEL_STACK_DECLARE instead.

() K_KERNEL_STACK_ARRAY_EXTERN
   Use K_KERNEL_STACK_ARRAY_DECLARE instead.

() K_KERNEL_PINNED_STACK_ARRAY_EXTERN
   Use K_KERNEL_PINNED_STACK_ARRAY_DECLARE instead.

() K_THREAD_STACK_EXTERN
   Use K_THREAD_STACK_DECLARE instead.

() K_THREAD_STACK_ARRAY_EXTERN
   Use K_THREAD_STACK_ARRAY_DECLARE instead.

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