ipn/ipnext: remove support for unregistering extension

Updates #12614

Change-Id: I893e3ea74831deaa6f88e31bba2d95dc017e0470
Co-authored-by: Nick Khyl <nickk@tailscale.com>
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f