contrib: debian: drop bashcompletion mangling after install

tested by running:

./configure --with-config=user; cp -a contrib/debian .
dpkg-buildpackage -b -uc -us

on a Debian 12 based system.

and checking where the completion file got installed.

Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Closes #15304