Fix duplicating "enable_transience" for fish

Starship enables fish transience automatically if it's transience is
enabled, duplicated call was causing errors

Remove obsolete fish.nix

system/home/shell/fish.nix

@@ -1,7 +0,0 @@
-{ ... }:
-{
-  programs.fish = {
-    enable = true;
-    interactiveShellInit = "enable_transience";
-  };
-}

system/home/shell/starship.nix

@@ -1,6 +1,5 @@
 { ... }:
 {
-  programs.fish.interactiveShellInit = "enable_transience";
   programs.starship = {
     enable = true;
     enableTransience = true;