tsweb: remove JSONHandlerFunc

It's unused and we've decided it's not what we want.

Change-Id: I425a0104e8869630b498a0adfd0f455876d6f92b
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