Removed conflict-ui prefix subcommand

I've moved it into a separate repository: https://gitlab.com/portmod/omwconflicts

While mildly useful, following the vein of !422 (merged) I think it makes more sense for it to be external. Since this has no exposed API other than the subcommand (which is a TUI), removing it shouldn't break anything. I'll add a package to openmw-mods shortly, so anyone still interested can install it through portmod and run it using portmod <prefix> run omwconflicts (currently identical to installing and running it outside of portmod, but following !422 (merged) the plan is to give each prefix a separate openmw.cfg, so it will integrate better if run within the sandbox as the sandbox will be configured to point to the prefix-specific openmw.cfg).