libgimpwidgets: issue #9673 screenshot plug-in crashes

In commit 48c27770 some unicode related changes were made. As a result of
that on Windows display_device, which was previously a duplicated string,
is now referenced directly.
However, the g_free was not removed, causing a crash.

We resolve this by removing the obsolete g_free.

libgimpwidgets/gimpwidgetsutils.c

@@ -714,7 +714,6 @@ gimp_monitor_get_color_profile (GdkMonitor *monitor)
             g_free (dir_utf16);
             g_free (filename);
             g_free (filename_utf16);
-            g_free (device_key);
           }
       }
   }