Delete the method _parse_other in the Ad operator class

[keileg]

The method pp.ad.Operator._parse_other() is almost surely never used in practice; rather it is a remnant of an older approach to the parsing. We should verify that it is actually not used, and purge if possible. This should not be a major task.

This will be a preparatory step towards a full rework of the Ad operator parsing. Though the purge can be done together with a larger rewrite, it is preferable to do it before, so that the code is a bit cleaner when we move to the larger part.