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

Modules/main.c

@@ -839,18 +839,6 @@ pymain_free(void)
     _PyPathConfig_ClearGlobal();
     _Py_ClearStandardStreamEncoding();
     _Py_ClearArgcArgv();
-#ifdef __INSURE__
-    /* Insure++ is a memory analysis tool that aids in discovering
-     * memory leaks and other memory problems.  On Python exit, the
-     * interned string dictionaries are flagged as being in use at exit
-     * (which it is).  Under normal circumstances, this is fine because
-     * the memory will be automatically reclaimed by the system.  Under
-     * memory debugging, it's a huge source of useless noise, so we
-     * trade off slower shutdown for less distraction in the memory
-     * reports.  -baw
-     */
-    _Py_ReleaseInternedUnicodeStrings();
-#endif /* __INSURE__ */
 }