Makefile: always delete hwdb sources

In eudev and systemd, we have code that deletes the hwdb sources from
the target - they are not useful since a binary hwdb is created from
them. However, if eudev or systemd is not used, then those sources are
not useful either. It's possible that other packages than eudev or
systemd install hwdb files, which would be left on the system.

Always remove the hwdb files.

Note that we don't expect much space savings from this, but anything may
help. It's certainly more consistent to do it always than just in eudev
and systemd.

We do this both from /usr/lib/udev (usual installation path for systemd)
and in /etc/udev (usual installation path for eudev) because packages
may install in either location.

We keep the comment explaining why it's done in rootfs-pre-cmd instead
of target-finalize - this was only present in eudev.mk.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>