gh-152238: Revert gh-150490 and gh-152200. (gh-152232)

Revert commits:
    gh-151593: Fix dead lock in PyDict insert_split_key() (#152200)
    gh-150490: Raise PyType_Modified for insertion into split dictionary (#150489)

For gh-150489, it violates locking discipline and results in deadlocks,
gh-151593 is an example of it being hit in CI.  The attempted fix 
gh-152200 avoids the deadlock but introduces a data-race.  The race
window is small but can be triggered with pure Python code.

Author: nascheme

Include/internal/pycore_dict.h

@@ -90,8 +90,6 @@ typedef struct {
 } PyDictUnicodeEntry;
 
 extern PyDictKeysObject *_PyDict_NewKeysForClass(PyHeapTypeObject *);
-extern void _PyDict_RemoveKeysForClass(PyHeapTypeObject *);
-extern void _PyDict_SplitKeysInvalidated(PyDictKeysObject* keys);
 extern PyObject *_PyDict_FromKeys(PyObject *, PyObject *, PyObject *);
 
 /* Implementations of the `|` and `|=` operators for dict, used by the
@@ -241,17 +239,6 @@ struct _dictkeysobject {
        see the DK_ENTRIES() / DK_UNICODE_ENTRIES() functions below */
 };
 
-struct _instancekeysobject {
-    PyTypeObject* dsk_owning_type;
-    struct _dictkeysobject dsk_keys;
-};
-
-static inline struct _instancekeysobject *_PyDictKeys_AsSharedKeys(struct _dictkeysobject *keys)
-{
-    assert(keys->dk_kind == DICT_KEYS_SPLIT);
-    return _Py_CONTAINER_OF(keys, struct _instancekeysobject, dsk_keys);
-}
-
 /* This must be no more than 250, for the prefix size to fit in one byte. */
 #define SHARED_KEYS_MAX_SIZE 30
 #define NEXT_LOG2_SHARED_KEYS_MAX_SIZE 6