[3.13] gh-122420: Fix accounting for immortal interned strings in refleak.py (GH-122421) (GH-122430)

The `_PyUnicode_Intern*` functions already adjust the total refcount, so
we don't want to readjust it in refleak.py.
(cherry picked from commit ac8da34621a574cd5773217404757a294025ba49)

Co-authored-by: Sam Gross <colesbury@gmail.com>