There were some problems about where to install `gio-launch-desktop` to
support multiarch systems without circular dependencies. Simon McVittie
suggested that, actually, given the current set of platforms supported
by `GDesktopAppInfo` (they’re all POSIX), we could just use `sh`.

That simplifies things nicely. `gio-launch-desktop` can always be
resurrected (and the multiarch debate continued and resolved) if needed
in future.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Fixes: #1633