Add a feature to delete all translated entries from the file in one go

Summary:
Feature requested on bugs.kde.org
This can be useful for a translator as you don't always actually have access to the .pot file on some commercial translation projects.

FEATURE: 389285

Reviewers: #localization, aacid, ltoscano, shaforostoff, mlaurent

Reviewed By: #localization, aacid

Tags: #localization

Differential Revision: https://phabricator.kde.org/D14683