Times change. There are several generators that will create code first entities and mappings using the EDMX as the source. There are several in the extensions library. The one you will most likely want to use is the EF 5.x DBContextFluent Generator for C#. http://visualstudiogallery.msdn.microsoft.com/5d663b99-ed3b-481d-b7bc-b947d2457e3c
There is also the EF 5.x DbContect Generator which iirc puts the mapping information on the models.
Once you have installed either of these you can edit the EDMX and right click "Add Code Generation Item". This will install the .tt templates and generate the Entity Class, DbContext class and mapping files.
Be aware that if you have any partials with validation if you had them in the same folder as your edmx with the same name as the entity they would be over written. You just need to rename those partials, since the generator does put the partial keyword on the entities, or better yet, move that code into the newly generated entity classes.
Once those files are in place you can delete the .tt files and the .edmx files and going forward you maintain your entities in code.