fix: drop full emacs runtime dep, only emacsclient is used

sims only invokes emacsclient (in modules/notmuch.py) and never the
full emacs binary. Bundling emacs onto the wrapper PATH was forcing
consumers to pull in an extra ~310M emacs closure (often a different
version from the user's actual editor). Rely on the consumer's
emacsclient via PATH, configurable via services.sims.settings.notmuch.emacsclient_command.

nix/derivation.nix

@@ -14,7 +14,6 @@
   webp-pixbuf-loader,
   notmuch,
   khal,
-  emacs,
   dbus,
   ...
 }:
@@ -74,11 +73,17 @@ python3Packages.buildPythonApplication {
 
   preFixup = ''
     makeWrapperArgs+=("''${gappsWrapperArgs[@]}")
-    makeWrapperArgs+=(--prefix PATH : ${lib.makeBinPath [ khal notmuch emacs dbus ]})
+    makeWrapperArgs+=(--prefix PATH : ${
+      lib.makeBinPath [
+        khal
+        notmuch
+        dbus
+      ]
+    })
   '';
 
   passthru = {
-    inherit khal notmuch emacs;
+    inherit khal notmuch;
   };
 
   meta = {