buildroot

package: Remove shell completion at finalize stage

Instead of doing a removal of the completion file package per package,
do it all at the finalize stage so it's done once and for all.

Note: This fixes an issue with systemd where passing a --bashcompletiondir
or --zshcompletiondir would be evaluated to '.' by the autotools macro.
This would create a 'target./' directory.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>