Fix off by one error in Cache::clear

Summary:
beginRemoveRows args are int first, int last

i.e If you have 1 entry you should be calling
beginRemoveRows(0, 0);

BUG: 396175

Test Plan:
Code review of a common mistake
Other usages in the same code were fine

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13895