py/objexcept: Remove long-obsolete mp_const_MemoryError_obj.

This constant exception instance was once used by m_malloc_fail() to raise
a MemoryError without allocating memory, but it was made obsolete long ago
by 3556e45711.  The functionality is now
replaced by the use of mp_emergency_exception_obj which lives in the global
uPy state, and which can handle any exception type, not just MemoryError.

py/obj.h

@@ -624,7 +624,6 @@ extern const struct _mp_obj_str_t mp_const_empty_bytes_obj;
 extern const struct _mp_obj_tuple_t mp_const_empty_tuple_obj;
 extern const struct _mp_obj_singleton_t mp_const_ellipsis_obj;
 extern const struct _mp_obj_singleton_t mp_const_notimplemented_obj;
-extern const struct _mp_obj_exception_t mp_const_MemoryError_obj;
 extern const struct _mp_obj_exception_t mp_const_GeneratorExit_obj;
 
 // General API for objects

py/objexcept.c

@@ -43,9 +43,6 @@
 // Number of traceback entries to reserve in the emergency exception buffer
 #define EMG_TRACEBACK_ALLOC (2 * TRACEBACK_ENTRY_LEN)
 
-// Instance of MemoryError exception - needed by mp_malloc_fail
-const mp_obj_exception_t mp_const_MemoryError_obj = {{&mp_type_MemoryError}, 0, 0, NULL, (mp_obj_tuple_t*)&mp_const_empty_tuple_obj};
-
 // Optionally allocated buffer for storing the first argument of an exception
 // allocated when the heap is locked.
 #if MICROPY_ENABLE_EMERGENCY_EXCEPTION_BUF

py/vm.c

@@ -1374,8 +1374,7 @@ unwind_loop:
             // set file and line number that the exception occurred at
             // TODO: don't set traceback for exceptions re-raised by END_FINALLY.
             // But consider how to handle nested exceptions.
-            // TODO need a better way of not adding traceback to constant objects (right now, just GeneratorExit_obj and MemoryError_obj)
-            if (nlr.ret_val != &mp_const_GeneratorExit_obj && nlr.ret_val != &mp_const_MemoryError_obj) {
+            if (nlr.ret_val != &mp_const_GeneratorExit_obj) {
                 const byte *ip = code_state->fun_bc->bytecode;
                 ip = mp_decode_uint_skip(ip); // skip n_state
                 ip = mp_decode_uint_skip(ip); // skip n_exc_stack