Updater's should stop iterating through ALL files in deleteOldFiles()

[kreativf]

This isn't so much of a bug, as it is a QOL improvement that might fix some stuff along the way.
Right now the deleteOldFiles() function will iterate through all files inside of the nextcloud folder, even though the files might be inside a directory listed in $excludedElements. In my case it takes forever to iterate through ALL of the data folder during each update, including the painfully large cache, even though at the end of the day it doesn't do anything to those files.

[joshtrichards]

Related: #507 (same solution will be likely used for both)

[come-nc]

Fixed by #516