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 openzfs#15304

contrib/debian/openzfs-zfsutils.install

@@ -1,7 +1,6 @@
 etc/default/zfs
 etc/zfs/zfs-functions
 etc/zfs/zpool.d/
-etc/bash_completion.d/zfs
 lib/systemd/system-generators/
 lib/systemd/system-preset/
 lib/systemd/system/zfs-import-cache.service

contrib/debian/rules.in

@@ -71,10 +71,6 @@ override_dh_auto_install:
 	@# Install the utilities.
 	$(MAKE) install DESTDIR='$(CURDIR)/debian/tmp'
 
-	# Use upstream's bash completion
-	install -D -t '$(CURDIR)/debian/tmp/usr/share/bash-completion/completions/' \
-		'$(CURDIR)/contrib/bash_completion.d/zfs'
-
 	# Move from bin_dir to /usr/sbin
 	# Remove suffix (.py) as per policy 10.4 - Scripts
 	# https://www.debian.org/doc/debian-policy/ch-files.html#s-scripts
@@ -136,7 +132,6 @@ override_dh_auto_install:
 
 	chmod a-x '$(CURDIR)/debian/tmp/etc/zfs/zfs-functions'
 	chmod a-x '$(CURDIR)/debian/tmp/etc/default/zfs'
-	chmod a-x '$(CURDIR)/debian/tmp/usr/share/bash-completion/completions/zfs'
 
 override_dh_python3:
 	dh_python3 -p openzfs-python3-pyzfs