notebook: fix invalid signal handler warning
This fixes this warning when closing tabs:
GLib-GObject-WARNING **: 20:45:54.015: ../../../../Projects/glib/gobject/gsignal.c:2647: instance '0x1a463b0' has no handler with id '7129'
It's caused because we accidentally disconnect a signal that's being managed by GtkNotebook.
Note: we're in hard code freeze, so this should be merged after 3.34.0 next week.