maint: Completely get rid of OVERRIDE_PACKAGING_DIR.

This is a follow-up to commit
47b1b0151d6c3703ee25c0050cd2031ab7edae4c, which removed it from the
make-packaging-target.py script.

* docker/Dockerfile_debian_10: Remove support code for
OVERRIDE_PACKAGING_DIR.
* docker/Dockerfile_debian_10_arm64: Likewise.
* docker/Dockerfile_debian_10_armhf: Likewise.
* docker/Dockerfile_debian_10_i386: Likewise.
* docker/Dockerfile_debian_10_qt: Likewise.
* docker/Dockerfile_ubuntu_18.04: Likewise.
* docker/Dockerfile_ubuntu_18.04_i386: Likewise.
* docker/Dockerfile_ubuntu_18.04_qt: Likewise.
* docker/Dockerfile_ubuntu_18.04_qt_i386: Likewise.
* docker/Dockerfile_ubuntu_20.04: Likewise.
* docker/Dockerfile_ubuntu_20.04_qt: Likewise.
* docker/Dockerfile_ubuntu_20.10: Likewise.
* docker/Dockerfile_ubuntu_20.10_qt: Likewise.
* scripts/build-package-debian-qt.sh: Likewise.
* scripts/build-package-debian.sh: Likewise.

Change-Id: Ia15961fe1ccc05222d43da2bdded819f95315aae