build-sys: use .DELETE_ON_ERROR

build-sys: use .DELETE_ON_ERROR

All instances of "|| rm $@" are replaced with .DELETE_ON_ERROR, which
has a similar effect. One difference is that the return code is not
masked by rm return code.

.DELETE_ON_ERROR is GNU-Make specific, but -Wno-portability is already
defined, and it's unlikely that anyone would build systemd with a
shell not supporting .DELETE_ON_ERROR. If they did, then
.DELETE_ON_ERROR would be silently ignored, i.e. in the worst case a
garbage file wouldn't be deleted, which is not very serious.