Cleanup files backup functionality

[matthijs-va]

As a follow-up to #6, the files option should be removed (where applicable) from the various backup methods. All other related functionality may be removed as well.

[daften]

@matthijs-va What do you mean with "All other related functionality".
And files should probably still be on when syncing to local e.g.

There's also still double functionality. e.g. in digipolisSyncLocal there's two sections for when $opt['files'] === true

[matthijs-va]

"All other related functionality" => Methods only used for the none-rsync sync.