We made changes to ipnext callback registration/unregistration/invocation in #15780
that made resetting b.exthost to a nil, no-op host in (*LocalBackend).Shutdown() unnecessary.
But resetting it is also racy: b.exthost must be safe for concurrent use with or without b.mu held,
so it shouldn't be written after NewLocalBackend returns. This PR removes it.
Fixes#17279
Signed-off-by: Nick Khyl <nickk@tailscale.com>