Revert "rootfs-postcommands.bbclass: add post func remove_unused_dnf_log_lock"

This was not well researched or explained, and obscures a problem elsewhere:
if dnf leaves lock files around, the problem should be fixed at the source,
and not in an after-the-fact function.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>