extmod/modbluetooth: Use MP_REGISTER_ROOT_POINTER().

This uses MP_REGISTER_ROOT_POINTER() to register `bluetooth`
instead of using a conditional inside of mp_state_vm_t.

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

extmod/modbluetooth.c

@@ -1675,4 +1675,6 @@ int mp_bluetooth_gatts_db_resize(mp_gatts_db_t db, uint16_t handle, size_t len,
     return entry ? 0 : MP_EINVAL;
 }
 
+MP_REGISTER_ROOT_POINTER(mp_obj_t bluetooth);
+
 #endif // MICROPY_PY_BLUETOOTH

py/mpstate.h

@@ -196,12 +196,6 @@ typedef struct _mp_state_vm_t {
     #include "genhdr/root_pointers.h"
     #endif
 
-    // root pointers for extmod
-
-    #if MICROPY_PY_BLUETOOTH
-    mp_obj_t bluetooth;
-    #endif
-
     //
     // END ROOT POINTER SECTION
     ////////////////////////////////////////////////////////////