Don't try to compute time stamps for files from Packet Builder.

I don't trust Packet Builder's ability to convert time stamps between
Capsa format and pcap.

Change-Id: I0ac2e14216e37127d81d5bf1c6d48a2c20841a8e
Reviewed-on: https://code.wireshark.org/review/4721
Reviewed-by: Guy Harris <guy@alum.mit.edu>