Partially revert commit ac46eb4:
"bpo-38113: Update the Python-ast.c generator to PEP384 (pythongh-15957)".

Using a module state per module instance is causing subtle practical
problems.

For example, the Mercurial project replaces the __import__() function
to implement lazy import, whereas Python expected that "import _ast"
always return a fully initialized _ast module.

Add _PyAST_Fini() to clear the state at exit.

The _ast module has no state (set _astmodule.m_size to 0). Remove
astmodule_traverse(), astmodule_clear() and astmodule_free()
functions..
(cherry picked from commit e5fbe0c)

Co-authored-by: Victor Stinner <vstinner@python.org>