[RichTextComposer] Remove stray backspace/delete handling code for headings

Summary:
The current code, unfortunately, resets rich text formatting for current line every time user presses backspace.
It could be safely removed, as the proper code which resets it only when needed was backported
as a part of 4092f7f0 commit.

Test Plan: backspace no longer resets formatting on a line with i.e. bold word

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D29627