wgengine/router/dns: remove unused code

Commit 68ddf1 removed code that reads
`SOFTWARE\Tailscale IPN\SearchList` registry value. But the commit
left code that writes that value.

So now this package writes and never reads the value.

Remove the code to stop pointless work.

Updates #853

Signed-off-by: Alex Brainman <alex.brainman@gmail.com>
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