device: remove QueueInboundElement leak with stopped peers

device: remove QueueInboundElement leak with stopped peers

This is particularly problematic on mobile,
where there is a fixed number of elements.
If most of them leak, it'll impact performance;
if all of them leak, the device will permanently deadlock.

I have a test that detects element leaks, which is how I found this one.
There are some remaining leaks that I have not yet tracked down,
but this is the most prominent by far.

I will commit the test when it passes reliably.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>