link_old_mar_filenames still referenced in torbrowser-incrementals-{release,alpha}-unsigned

We still reference that script, but it's been removed a while ago and it makes the build fail:

> make torbrowser-incrementals-alpha-unsigned
git submodule update --init
./rbm/rbm build release --step update_responses_config --target alpha --target unsigned_releases_dir --target torbrowser
./rbm/rbm build release --step link_old_mar_filenames --target alpha --target unsigned_releases_dir --target torbrowser
Error: Missing link_old_mar_filenames config
make: *** [Makefile:203: torbrowser-incrementals-alpha-unsigned] Error 1