packaging: remove hg-ssh.8.html from Inno installer
We don't ship hg-ssh because it requires a python.exe to
run, which we don't ship. So it doesn't make sense to ship
the HTML documentation for this tool.
This change makes the Inno install layout more consistent
with WiX, which doesn't ship this file.
Functionality for removing files has been made generic,
in anticipation of future expansion.
Differential Revision: https://phab.mercurial-scm.org/D7169