bt2: remove BT CC entry from global HT in _UserComponentType.__del__()

bt2: remove BT CC entry from global HT in _UserComponentType.__del__()

The global `bt_cc_ptr_to_py_cls` hash table maps BT component class
pointers to Python component classes (`PyObject *`) for component
classes created in Python. The key and value are weak references.

When the Python side calls native_bt.bt2_component_class_*_create() with
a Python component class, an entry is added to `bt_cc_ptr_to_py_cls` on
success. This works most of the time because all Python classes are
normally destroyed when the interpreter is finalized, but there could be
issues with specific/unusual import patterns.

To avoid potential issues, remove an entry from `bt_cc_ptr_to_py_cls` in
_UserComponentType.__del__(), where a strong BT component class
reference is finally released.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I450a1c6f179f352e758b9e3e5cac8f4711aa3c88
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1816
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>