Manage session variables with home-manager

If you haven't noticed, home-manager is managing increasingly more of my
configuration.

- Migrate session variables to home.nix
- Drop support for unused session variables like TERMINAL, VISUAL

configs/.config/nixpkgs/home.nix

@@ -23,6 +23,9 @@
       ];
     };
     sessionVariables = {
+      BROWSER = "google-chrome";
+      EDITOR = "emacsclient";
+      ALTERNATE_EDITOR = "vim";
     };
     stateVersion = "19.09";
   };