ipn: remove handle.go

It was unused in this repo. The Windows client used it, but it can move there.

Change-Id: I572816fd80cbbf1b8db734879b6280857d5bd2a7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f