Remove some unnecessary definitions.

text2pcap.c uses pcapio.c to write pcap files, so it doesn't itself need
to know what those files look like.

Change-Id: I013d0c094c27515af91f352b565bc9a8bad3e842
Reviewed-on: https://code.wireshark.org/review/3927
Reviewed-by: Guy Harris <guy@alum.mit.edu>