Removal of freemarker.debug package

[Daniel Dekany <dd...@apache.org>]

We plan to remove the freemarker.debug package in the next release
(2.3.35), because it uses RMI (a legacy Java remote method call protocol),
which is for a while seen as a security concern. While this feature is not
active by default (you had to start a debugger client, and a debugger
server explicitly), the mere presence of these classes is flagged at some
places.

I believe practically nobody uses the debugger feature, that's why even
though it breaks backward compatibility, I guess it's an OK compromise to
do this now. But I wanted to communicate this, just in case someone has
problems with this, or needs to prepare for this.