I know for a calculated field on a custom form, you can use...
WEEKDAYDIFF(Entry Date,Actual Completion Date,"521504260091b36b47f167f8571ae01c")
The ID number is the Workfront ID of the Schedule whose holidays you don't want counted.
However, I have never tried this in a report as a column so not sure if this will work...
WEEKDAYDIFF({entryDate},{actualCompletionDate},"521504260091b36b47f167f8571ae01c")