Revert "Consider slow files as remote files in previewjob"

This reverts commit afb37351.

This is the same issue for any remote filesystem, nfs, samba, sshfs, that is mounted locally (rather than accessed using kio without mounting) all such filesystem are treated as local by upstream QFile* API, i.e. file:// scheme; there isn't much we can do about that. So users can disable previews to make things work for really slow filesystems...

BUG: 450482 CCBUG: 349161 CCBUG: 372849

FIXED-IN: 5.94