Remove 4 private trashcan C API functions which were only kept for the backward
compatibility of the stable ABI with Python 3.8 and older, since the trashcan
API was not usable with the limited C API on Python 3.8 and older. The
trashcan API was excluded from the limited C API in Python 3.9.

Removed functions:

* _PyTrash_deposit_object()
* _PyTrash_destroy_chain()
* _PyTrash_thread_deposit_object()
* _PyTrash_thread_destroy_chain()

The trashcan C API was never usable with the limited C API, since old trashcan
macros accessed directly :c:type:`PyThreadState` members like
``_tstate->trash_delete_nesting``, whereas the :c:type:`PyThreadState`
structure is opaque in the limited C API.

Exclude also the the ``PyTrash_UNWIND_LEVEL`` constant from the C API.

Patch by Victor Stinner.