How do I remove these app-files?

The short answer is "you don't". Let Time Machine manage itself. Do not attempt to interfere with it.

• When a Time Machine backup disk becomes full or nearly so (which is normal) Time Machine will begin to delete old, "expired" backups on its own. Never attempt to do that yourself.

It is unclear from that screenshot whether those files exist in that Time Capsule's backups or not. Normally, the Time Machine backup disk on a Time Capsule is named "Backup". If you changed that name to "Software" that's ok, but the fact there is a subfolder within it named "Old files that I can't delete" indicates you may have performed unsupported modifications to the Time Machine backup disk. That action is 100% certain to corrupt the backup, rendering it useless. In that event the only solution is to erase the backup disk. Use AirPort Utility to do that: AirPort Utility User Guide for Mac - Apple Support. Select the Time Capsule, Edit > Disks, Erase Disk...