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";
   };

configs/.profile

@@ -19,13 +19,6 @@ if [ -z "$DISPLAY" -a "$(tty)" = '/dev/tty5' ]; then
   exec xinit -- vt05
 fi
 
-# Application preferences
-export BROWSER=google-chrome-stable
-export TERMINAL=alacritty
-export VISUAL=emacsclient
-export EDITOR=emacsclient
-export ALTERNATE_EDITOR=nvim
-
 # Rust integration. Keep here so programs like Emacs can read this value.
 # export RUST_SRC_PATH="$(rustc --print sysroot)/lib/rustlib/src/rust/src"