Argument Clinic: add support for removing a parameter #108270

Originally posted by @serhiy-storchaka in #95065 (comment):

3. Remove parameter. For some converters we can use NULL or something like to distinguish absent argument, but for others all values after conversion are correct, so we need a special directive for this. When remove positional-only or positional-or-keyword parameter, all following parameters should become keyword-only parameters.