Setup.hs: Stop installing man pages, desktop files, and the git-annex-shell and git-remote-tor-annex symlinks

Anything still relying on that, eg via cabal v1-install will need to
change to using make install-home. Which was added back in 2019 in
6491b626 because cabal new-build
(now the default) already didn't use Setup in a way that let its
installation of those things work.

Notably this means Setup does not need to depend on unix-compat, which is
useful because in 0.7 it removed System.PosixCompat.User, which Setup
needed to determine where to install the desktop files. See
https://github.com/haskell-pkg-janitors/unix-compat/issues/3