rootfs-postcommands.bbclass: add post func remove_unused_dnf_log_lock

Remove log_lock.pid which maybe created during do_rootfs. In commit
[dnf: only write the log lock to root for native dnf],
native dnf changed to write log lock to root, and target dnf still
use /var/log, so log_lock.pid need to be removed post do_rootfs.

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>