[issue31377] remove *_INTERNED opcodes from marshal

[INADA Naoki]

INADA Naoki added the comment:

> We end up interning each reference individually currently.

But interning interned string is much faster. It only checks flag.
Interning normal string requires dict lookup.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue31377>
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