multimedia/mythtv: revise pkg-plist after exiv2 0.28 upgrade

mythtv used to expect exiv2 0.27.99 or newer, and would replace
and install a local copy of exiv2 if missing.

With exiv2 0.28 in the ports tree, mythtv will pick it up from
the system, and we need to purge the relevant files from pkg-plist.

PR: 272311