machine/wsl: unregister shared net-usermode dist on machine rm

[Srujansa]

Fixes #29480

The podman-net-usermode WSL distribution is shared across all
machines using --user-mode-networking, but podman machine rm
never unregistered it — only the machine's own WSL distro was
removed, leaving podman-net-usermode behind indefinitely.

This adds a check in WSLStubber.Remove(): if the removed machine
had user-mode networking enabled, it now looks at the other
configured machines and only unregisters podman-net-usermode if
none of them still need it.

Tested manually on Windows:

• single machine with --user-mode-networking: podman machine rm
  now also removes podman-net-usermode from wsl --list
• two machines both using --user-mode-networking: removing one
  leaves podman-net-usermode intact for the other