nginx: fix nginx.old clean-up

The bin path for nginx is configured for /usr/sbin, so deleting
the nginx.old back-up from /usr/bin never works.  Fix path, and
also use "$(RM)" instead of "-$(RM)", as "rm -f" never fails.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Tested-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>