bpo-39465: Remove _PyUnicode_ClearStaticStrings() from C API (GH-20078)

Remove the _PyUnicode_ClearStaticStrings() function from the C API.
Make the function fully private (declare it with "static").
diff --git a/Include/cpython/object.h b/Include/cpython/object.h
index 8bf05a3..444f832 100644
--- a/Include/cpython/object.h
+++ b/Include/cpython/object.h
@@ -36,7 +36,7 @@
 
    PyId_foo is a static variable, either on block level or file level. On first
    usage, the string "foo" is interned, and the structures are linked. On interpreter
-   shutdown, all strings are released (through _PyUnicode_ClearStaticStrings).
+   shutdown, all strings are released.
 
    Alternatively, _Py_static_string allows choosing the variable name.
    _PyUnicode_FromId returns a borrowed reference to the interned string.