ipn/ipnlocal: don't run portlist code unless service collection is on

We were selectively uploading it, but we were still gathering it,
which can be a waste of CPU.

Also remove a bunch of complexity that I don't think matters anymore.

And add an envknob to force service collection off on a single node,
even if the tailnet policy permits it.

Fixes #13463

Change-Id: Ib6abe9e29d92df4ffa955225289f045eeeb279cf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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