Backspace without CTRL should send '^?'

Summary:
This matches XTerm behaviour, so that pressing Alt+Backspace will send
\E + Del, which is the expected behaviour in some apps (e.g. emacs),
and it also matches the Konsole behaviour before commit
5345bbb0

BUG: 394276
FIXED-IN: 18.04.2

Test Plan:
- Make sure you have the default .keytab file without any modifications
- Run `emacs -Q -nw`, press Ctrl+h k, to invoke the describe key functionality
- Press Alt+Backspace, it'll send C-M-h
- Apply the diff and test again, now it should send M-Del

Reviewers: #konsole, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: konsole-devel, #konsole

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D12899