multimedia/openh264: clean up pkg-plist

cliqz has been removed 2020-12-31, and seamonkey was removed 2019-07-13.
A firefox-nightly might even never have existed in the official tree
(it sure is not there riht now, and I cannot find any commit related
to that in the git history).
No PORTREVISION bump, as this does not change any installed file, and
the symlinks created/removed by the postexec statements shouldn't exist
in any remotely current system.