Remove instructions to disable SSH HostKeyChecking or clarify use.

Created by: Nilos

https://github.com/gitlabhq/gitlabhq/blob/stable/doc/install/installation.md#disable-stricthostkeychecking-for-localhost-and-your-domain

This part tells everyone to disable stricthostkeychecking, which is there for a good reason: to prevent man in the middle attacks.

There should at least be a part explaining why this should be disabled. It would be best to just remove it, as I could not find any reason to do so.