notmuch: inline `notmuch-accounts.nix`

Having it in a separate file is a bit unnecessary.

modules/programs/notmuch-accounts.nix

@@ -1,5 +0,0 @@
-{ lib, ... }:
-
-{
-  options.notmuch = { enable = lib.mkEnableOption "notmuch indexing"; };
-}

modules/programs/notmuch.nix

@@ -145,7 +145,10 @@ in {
     };
 
     accounts.email.accounts = mkOption {
-      type = with types; attrsOf (submodule (import ./notmuch-accounts.nix));
+      type = with types;
+        attrsOf (submodule {
+          options.notmuch.enable = mkEnableOption "notmuch indexing";
+        });
     };
   };