Fixes minor memory leak in the KNSBackend

Summary:
Fixes minor memory leak in the KNSBackend, the destructor of KNSBackend
failed to delete the contents of the m_rootCategories vector.

Test Plan:
Build with -fsanitize=address
Open and close Discover

Reviewers: apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D15145