control/controlclient: merge, simplify two health check calls

I'm trying to remove some stuff from the netmap update path.

Updates #1909

Change-Id: Iad2c728dda160cd52f33ef9cf0b75b4940e0ce64
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