client/tailscale/apitype, tailcfg: delete never used mysterious PerDomain field

It does nothing and never did and I don't think anybody remembers what
the original goal for it was.

Updates #5229 (fixes, but need to clean it up in another repo too)

Change-Id: I81cc6ff44d6d2888bc43e9145437f4c407907ea6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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