control/controlclient: remove dummy endpoint in endpoint stripping mode

The TODO is done. Magicsock doesn't require any endpoints to create an
*endpoint now.  Verified both in code and empirically: I can use the
env knob and access everything.

Change-Id: I4fe7ed5b11c5c5e94b21ef3d77be149daeab998a
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