move manpages to maintainer-clean make target

according to https://www.gnu.org/prep/standards/html_node/Standard-Targets.html#Standard-Targets
`maintainer-clean` is the proper make target for files that are distributed
in a release tarball:

> The ‘maintainer-clean’ target is intended to be used by a maintainer of the
> package, not by ordinary users.
> You may need special tools to reconstruct some of the files that
> ‘make maintainer-clean’ deletes.

this prevents users without a2x or asciidoctor from losing their ability to
recompile tinyproxy after `make clean`, but it also means that users wanting
to regenerate the documentation need to run `make maintainer-clean`.

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