cmd/tailscale: remove TS_EXPERIMENT_OAUTH_AUTHKEY guardrail

We've had support for OAuth client keys in `--authkey=...`
for several releases, and we're using it in
https://github.com/tailscale/github-action

Remove the TS_EXPERIMENT_* guardrail, it is fully supported now.

Fixes https://github.com/tailscale/tailscale/issues/8403

Signed-off-by: Denton Gentry <dgentry@tailscale.com>
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