Generally speaking, you can remove any file under /opt/kaltura/app/cache but you shouldn’t remove directories because these must be set with specific permissions/ownership to allow the various Kaltura daemons [Batch, Apache, Sphinx, etc] to write to them.
On a side note, please bear in mind that by default, when you delete content [using the management I/Fs or the API directly], the actual media files are NOT removed and so, no disk space will be freed. When calling the
delete() action, the entry.status DB column will simply be set to 3 [KalturaEntryStatus::DELETED]. So, in other words, it is a “soft” deletion operation. This is done in order to accommodate content restoration.
That behaviour can be changed by going to Admin Console->Publishers->Your partner->Actions->Configure and enabling the “Purge files on deletion” option.