• Fix a misbinding of <Bsp>

When curses gives no code for Ctrl+Shift+Delete, do not fall back
to KEY_BACKSPACE, because then ^H and/or <Backspace> get bound to
'cutwordleft'.

This fixes https://savannah.gnu.org/bugs/?54642.

Bug was introduced with version 3.0, commit e6429e78.

Reported by: eadler