esp32/mpthreadport: Don't explicitly free thread struct in TCB cleanup.

Because vPortCleanUpTCB runs on the FreeRTOS idle task and cannot execute
any VM or runtime related code like freeing memory.

Signed-off-by: Damien George <damien@micropython.org>

ports/esp32/mpthreadport.c

@@ -166,6 +166,8 @@ void mp_thread_finish(void) {
     mp_thread_mutex_unlock(&thread_mutex);
 }
 
+// This is called from the FreeRTOS idle task and is not within Python context,
+// so MP_STATE_THREAD is not valid and it does not have the GIL.
 void vPortCleanUpTCB(void *tcb) {
     if (thread == NULL) {
         // threading not yet initialised
@@ -182,8 +184,7 @@ void vPortCleanUpTCB(void *tcb) {
                 // move the start pointer
                 thread = th->next;
             }
-            // explicitly release all its memory
-            m_del(thread_t, th, 1);
+            // The "th" memory will eventually be reclaimed by the GC.
             break;
         }
     }