keyboard: Specify anchor when deleting characters via timeout

keyboard: Specify anchor when deleting characters via timeout

6e80f858c dropped the second cursor argument used as anchor when calling
the delete function from the timeout. This was causing the early check
that was trying to detect when deleting from the beginning of the text
to fail, because anchor was now undefined.

Fixes: 6e80f858c ("keyboard: Move backspace key handling to KeyboardController")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7484
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3239>