Use new display string function from KFileMetaData

Summary:
Delete all the custom formatting functions
and use the ones provided from KFileMetaData.

Reviewers: #dolphin, elvisangelaccio

Reviewed By: #dolphin, elvisangelaccio

Subscribers: bcooksley, elvisangelaccio, kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D19105