build: Fix automagic dependency on ytnef library

It would be best to use meson feature option type for ytnef and unwind, but for now fix the automagic dependency on ytnef with the existing meson_options.txt setup. It was adding ytnef to list of dependencies unconditionally, and then also if tnef-support was enabled. So if the option was disabled, but the pkg-config file was present, it would still link to it surprisingly. Just remove the unconditional addition to dep, and rely on the already existing conditional addition to deps.