Revert tcpdump to using the source manpage instead of having a copy here.
This helps future maintainability of tcpdump so we don't forget to update
the manpage (like we have previously).
Stolen from: usr.bin/file/Makefile
Reviewed by: jilles
Approved by: delphij (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D12403