py/scheduler: Move clearing of kbd traceback to mp_keyboard_interrupt.

This is a more logical place to clear the KeyboardInterrupt traceback,
right before it is set as a pending exception.  The clearing is also
optimised from a function call to a simple store of NULL.

lib/utils/interrupt_char.c

@@ -32,9 +32,6 @@
 int mp_interrupt_char = -1;
 
 void mp_hal_set_interrupt_char(int c) {
-    if (c != -1) {
-        mp_obj_exception_clear_traceback(MP_OBJ_FROM_PTR(&MP_STATE_VM(mp_kbd_exception)));
-    }
     mp_interrupt_char = c;
 }
 

ports/samd/mphalport.c

@@ -39,9 +39,6 @@ void tud_cdc_rx_wanted_cb(uint8_t itf, char wanted_char) {
 }
 
 void mp_hal_set_interrupt_char(int c) {
-    if (c != -1) {
-        mp_obj_exception_clear_traceback(MP_OBJ_FROM_PTR(&MP_STATE_VM(mp_kbd_exception)));
-    }
     tud_cdc_set_wanted_char(c);
 }
 

py/scheduler.c

@@ -29,7 +29,9 @@
 #include "py/runtime.h"
 
 #if MICROPY_KBD_EXCEPTION
+// This function may be called asynchronously at any time so only do the bare minimum.
 void MICROPY_WRAP_MP_KEYBOARD_INTERRUPT(mp_keyboard_interrupt)(void) {
+    MP_STATE_VM(mp_kbd_exception).traceback_data = NULL;
     MP_STATE_VM(mp_pending_exception) = MP_OBJ_FROM_PTR(&MP_STATE_VM(mp_kbd_exception));
     #if MICROPY_ENABLE_SCHEDULER
     if (MP_STATE_VM(sched_state) == MP_SCHED_IDLE) {