Fix some unused variable warnings in GCC 14.2.1 output

Reduce build noise a little bit.

app/tests/test-ui.c

@@ -174,7 +174,6 @@ keyboard_zoom_focus (gconstpointer data)
   Gimp              *gimp    = GIMP (data);
   GimpDisplay       *display = GIMP_DISPLAY (gimp_get_display_iter (gimp)->data);
   GimpDisplayShell  *shell   = gimp_display_get_shell (display);
-  GimpImageWindow   *window  = gimp_display_shell_get_window (shell);
   gint               image_x;
   gint               image_y;
   gint               shell_x_before_zoom;

app/widgets/gimpdockwindow.c

@@ -995,7 +995,6 @@ gimp_dock_window_image_changed (GimpDockWindow *dock_window,
                                 GimpImage      *image,
                                 GimpContext    *context)
 {
-  GimpContainer *image_container   = dock_window->p->image_container;
   GimpContainer *display_container = dock_window->p->display_container;
 
   /*  make sure auto-follow-active works both ways  */

plug-ins/gradient-flare/gradient-flare.c

@@ -5037,9 +5037,7 @@ gradient_get_values_real_external (const gchar *gradient_name,
   GimpGradient  *gradient;
   GeglColor    **colors;
   const Babl    *format = babl_format ("R'G'B'A u8");
-  gdouble      *tmp_values;
   gint          i;
-  gint          j;
 
   gradient = gimp_gradient_get_by_name (gradient_name);