Use KFormat for formatting byte sizes

Summary:
Use what is already available in KCoreAddons, instead of two custom
versions of it.

Test Plan: Builds fine, and the standard formatting is used.

Reviewers: Krita, dkazakov

Reviewed By: Krita, dkazakov

Subscribers: dkazakov, alvinhochun

Tags: Krita

Differential Revision: https://phabricator.kde.org/D13548