bpo-36356: Release Unicode interned strings on Valgrind (#12431)

When Python is compiled with Valgrind support, release Unicode
interned strings at exit in _PyUnicode_Fini().

* Rename _Py_ReleaseInternedUnicodeStrings() to
  unicode_release_interned() and make it private.
* unicode_release_interned() is now called from _PyUnicode_Fini():
  it must be called with a running Python thread state for TRASHCAN,
  it cannot be called from pymain_free().
* Don't display statistics on interned strings at exit anymore