Remove the option ‘programs.bash.enable’

NixOS has a pervasive dependency on bash. For instance, the X11
session script sources /etc/profile to get a reasonable
environment. Thus we should not provide an option to disable bash.

Also, enabling zsh no longer sets ‘users.defaultUserShell’ to zsh, to
prevent a collision with bash's definition of the same
option. (Changing the default shell is also something that should be
left to the user.)

nixos/modules/programs/zsh/zsh.nix

@@ -168,7 +168,7 @@ in
 
     environment.systemPackages = [ pkgs.zsh ];
 
-    users.defaultUserShell = mkDefault "/run/current-system/sw/bin/zsh";
+    #users.defaultUserShell = mkDefault "/run/current-system/sw/bin/zsh";
 
     environment.shells =
       [ "/run/current-system/sw/bin/zsh"