This patch refactors gst_wl_display_callback_destroy() to use the
recently introduced gst_wl_display_object_destroy() helper. Previously,
the function manually handled wl_callback destruction with explicit
lock/unlock calls and direct invocation of wl_callback_destroy().
Switching to gst_wl_display_object_destroy() unifies the destruction
process across similar objects, reducing code duplication and potential
errors.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8242>
a99694cca8