file extraction: always prune files after detect

file extraction: always prune files after detect

If a keyword like filemd5 was being used without a filestore,
or a file output enabled, it would be pruned before detection
had a chance to match.

Consolidate file pruning to the end of the flow worker so files
are available for detection even when a file output is not
enabled.

Redmine issue:
https://redmine.openinfosecfoundation.org/issues/2490