How to hide "Manage Fields" and "Manage Display" tabs
I know this was answered a while back, but there's a solution that doesn't require any custom or special modules.

If you disable the core fields ui module, the Manage Fields and Manage Display tabs will not show. 

If this is a production site, this should be fine to do as you'll have already configured the fields how you want them. If you ever need to make a change, just enable the module again briefly.