Remove unused option from ldap configuration in config.json.example

[thegcat]

From what I can tell (I am not a node nor a js dev) the tokenSecret key in the ldap configuration is not used anymore, is this correct? I would suggest removing it from the example configuration file and from the environment variables.

https://github.com/hackmdio/hackmd/blob/master/config.json.example#L77

[SISheogorath]

I checked and yes, it's not passed anywhere. But I'm not sure it's by accident or intended.

Maybe the devs of the feature can tell a bit more? @alecdwm @neopostmodern

[thegcat]

It was used when it was introduced 02e9927#diff-45ced1ab657dd3dcf5d098a56bc0f039R138

I don't see anything looking remotely like that in the current code anymore :-)

[SISheogorath]

Yes, this was changed a while back, so it should no longer be needed.

The re-factoring of the auth system solved a lot of these problems ^^

[SISheogorath]

Feel free to provide a PR 👍

[thegcat]

Done in #755, thanks!