ipn/ipnlocal: fix build, remove another Notify.BackendLogID reference that crept in

I merged 5cae7c51bf (removing Notify.BackendLogID) and 93db503565
(adding another reference to Notify.BackendLogID) that didn't have merge
conflicts, but didn't compile together.

This removes the new reference, fixing the build.

Updates #14129

Change-Id: I9bb68efd977342ea8822e525d656817235039a66
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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