Inactive maintainer package orphaning (part 3)

[Jon Turney]

I've orphaned the following packages:

cygcheck-dep                                 Mikhail Usenko
cygextreg                                    Joni Eskelinen
engauge-digitizer                            Mark Mitchell
polipo                                       Alexander Georgievskiy

The maintainers of which have been inactive for a long time (no activity 
since 2020), and didn't respond to my attempts to contact them.

Consequently, these maintainers' (long unused) ssh keys are also disabled.

polipo is unmaintained and considered obsolete upstream, and 
engauge-digitizer upstream seems to have disappeared, so maybe both of 
those are candidates for removal.