HACKING: Drop from the git repository

HACKING: Drop from the git repository

Despite being a generated file, HACKING has been tracked in
the git repository along with actual source files. As far as
I'm aware, it's the only generated file for which that happens.

Times and times again, people[1] have committed changes to
the source file without refreshing the generated copy at the
same time.

The rationale for tracking the generated file is to help out
people who just cloned the git repository looking to contribue;
however, README-hacking already contains enough information to
get perspective contributors to a place where they can simply
look at docs/hacking.html instead.

[1] Mostly me, to be honest

Signed-off-by: Andrea Bolognani <abologna@redhat.com>