In 2017, the dpkg suite introduced the rootless builds support with the
following commits:

  - 2436807c87b0 ("dpkg-deb: Add support for rootless builds")
  - fca1bfe84068 ("dpkg-buildpackage: Add support for rootless builds")

This feature is available in the default dpkg on Debian 10 and Ubuntu
20.04.

Remove the old method.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>