how can i "gc" or "prune" objects related to a deleted remote?

  writing a short tutorial on how to add a remote and work with it
and, as a final step, show how, if one is uninterested in the remote
after all, one can simply delete it, but i also want to show how one
can then prune or garbage collect the objects related to that remote,
but i can't figure out how.

  as an example, i cloned the linux kernel source tree, then added
the linux-next repo as a remote -- the end result was two pack files
(the smaller one i'm assuming being for linux-next):

$ ls -l .git/objects/pack total 2723632 -r--r--r--. 1 rpjday rpjday 1215376 Mar 8 09:44 pack-08cc266c0914e924961a1c8412fdf8746d327d7e.idx -r--r--r--. 1 rpjday rpjday 38402840 Mar 8 09:44 pack-08cc266c0914e924961a1c8412fdf8746d327d7e.pack -r--r--r--. 1 rpjday rpjday 204032716 Mar 8 09:42 pack-1036510bb74967c91093fc0cd8982683a66dbf5f.idx -r--r--r--. 1 rpjday rpjday 2545333327 Mar 8 09:42 pack-1036510bb74967c91093fc0cd8982683a66dbf5f.pac $

  after playing with a couple branches from the new remote, i deleted
the remote, then also did things like clear the reflog, delete any
local tracking branches related to the deleted remote, and so on, but
i can't seem to prune the objects that should no longer be reachable
now that i deleted that remote.

  what am i overlooking? is it because those objects are in a separate
pack file? do i need to repack first? what is hanging onto those
objects in that second pack file such that they can't be garbage
collected?

rday