fix: remove startup warning with local tailscaled

[dkaser]

This removes the startup warning that labels -use-local-tailscaled as development-only and advises against production use.

The warning was initially added due to concerns that identity could be forged using XFF headers when tsidp was running in local-tailscaled mode. That concern was resolved via #179.

-use-local-tailscaled is beneficial for production environments because it allows for embedded installations of tsidp. For example, see https://github.com/dkaser/unraid-tsidp , which generates a local config file for tsidp, starts tsidp in local-tailscaled mode, and configures Unraid to use tsidp as an IdP. After installation, the only thing that a user has to do is add the authorized users to the Unraid configuration, and then they can log in with Tailscale.

Using tsnet mode would increase user friction and create additional failure modes for embedded installations:

• Users have to either provide an auth key or do a separate login using the auth URL from the logs
• Tailscale assigning a different hostname to the tsidp node on join (e.g., due to conflict with an existing instance / previous install), which the user would then have to correct/provide
• Connectivity issues between the appliance and the tsidp instance it creates (e.g., if the user tags their appliance, but fails to tag the running tsidp instance or otherwise has the communication between the two blocked by policy)