preseed_base: Remove reportbug-gtk if present

preseed_base: Remove reportbug-gtk if present

The Debian installer have started to install "reportbug-gtk" which
depends on "dconf-service" and "dbus-user-session" and "systemd".

And when we try to install "sysvinit-core" we are met with the
following error message:
>    The following packages have unmet dependencies:
>     dconf-service : Depends: default-dbus-session-bus or
>			       dbus-session-bus

So, remove that new package and its dependencies before moving on.

While here, add `set -x` to help debugging.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>