Remove FSUtils::getDirectoryFileSystem

Summary:
It was only used to determine if the filesystem is BTRFS and disable
CoW if it is, which was incomplete and wrong.
If there should ever arise the need to do something in a filesystem
dependent manner, better use the portable QStorageInfo.

Test Plan: make

Reviewers: Baloo, FreeBSD, michaelh

Reviewed By: Baloo, michaelh

Subscribers: Frameworks

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D12138