VE-1551: Removed "Recently edited" sort functionality from Category Exhibition

Along with removing the option from the UI method (getRecentlyEdited) used for getting data from database was removed as well.
Reason for removing is that query is not performant (temporary table and filesort is being used) but also functionality is not popular/important enough to fix it.

https://wikia-inc.atlassian.net/browse/VE-1551