Safe Browsing configuration: improve wording, remove redundant GroupBox

Summary:
This de-clutters the **Configure Internal Browser Component** dialog a bit by:
- removing the redundant **Safe Browsing** GroupBox
- improving the wording of the Safe Browsing option

== Before/After
{F3733787, size=full}

Test Plan:
- Applied diff to Akregator build
- Opened configuration dialog
- Enabled SafeBrowsing
- Navigated to http://malware.testing.google.test/testing/malware/

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: #kde_pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D5563