py/mpstate: Drop MICROPY_PORT_ROOT_POINTERS from mp_state_vm_t.

All in-tree uses of MICROPY_PORT_ROOT_POINTERS have been replaced with
MP_REGISTER_ROOT_POINTER(), so now we can remove both
MICROPY_PORT_ROOT_POINTERS and MICROPY_BOARD_ROOT_POINTERS from the code
and remaining config files.

Signed-off-by: David Lechner <david@pybricks.com>

py/mpstate.h

@@ -189,9 +189,6 @@ typedef struct _mp_state_vm_t {
     mp_obj_t track_reloc_code_list;
     #endif
 
-    // include any root pointers defined by a port
-    MICROPY_PORT_ROOT_POINTERS
-
     // Include any root pointers registered with MP_REGISTER_ROOT_POINTER().
     #ifndef NO_QSTR
     // Only include root pointer definitions when not doing qstr extraction, because