remove pcap.spec (GH #384)

The file does nothing because RPM-based OS distributions have developed
their own .spec files to build the libpcap .rpm in accordance to their
standard (let alone dpkg and ports). There is no reasonable way and
sense to outperform them in that, yet there is a number of other
outstanding problems, thus I find it best to remove the file.

Those looking to produce a custom OS package of libpcap should base
their work on one of the many existing libpcap packages.