[KJotsEdit] Don't mess with margins when pasting rich text

Summary:
Original code contained weird html parsing, removing `margin-*: *px;' style properties from raw HTML.
However, when inserted, QTextDocument sees no margins and decides to set them to the default 12px,
which breaks even copy-pasting of non-formatted text (see attached).

Instead just use standard QTextDocument manipulation methods to remove margins. It fixes both pasting
formatted and non-formatted text.

BUG: 302107

Test Plan:
Before:
After:

Reviewers: dvratil, mlaurent

Reviewed By: mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D29177