keyboard: Fix backspace not getting released after deleting preedit

keyboard: Fix backspace not getting released after deleting preedit

When deleting is toggled while a preedit string is present, this causes
_toggleDelete() to use keyvalPress(). After the entire preedit string
has been deleted, _toggleDelete() will no longer hit the code path that
would call keyvalRelease() due to hasPreedit() now being false.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/7171
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3009>