No duplicate linear predecessors

[Procyonae]

Summary

None

Purpose of change

Replaces the hacky function I added in #67223 with a better and consistent solution
Needed for #67470

Describe the solution

Prevents the duplicates ever getting added rather than dealing with them when they enter reality bubble
I tried making predecessors_ a cata::flat_set too but couldn't figure out the savegame stuff, I were trying to do something vaguely like:

} else if( name == "predecessors" ) {
           auto flattened_predecessors;
            om_member.read( flattened_predecessors, true );

            void read_pred [&] ( std::vector<std::pair<tripoint_om_omt, std::vector<oter_id>>> flattened_predecessors ) {
                for( std::pair<tripoint_om_omt, std::vector<oter_id>> &p : flattened_predecessors_old ) {
                    predecessors_.insert( std::move( p ) );
                }
            };

            void read_pred [&] ( std::vector<std::pair<tripoint_om_omt, cata::flat_set<oter_id>>> flattened_predecessors ) {
                for( std::pair<tripoint_om_omt, std::vector<oter_id>> &p : flattened_predecessors ) {
                    predecessors_.insert( p );
                }
            };

           read_pred( flattened_predecessors );
        }

(L657 of src/savegame.cpp)

but it doesn't like the uninitialised auto flattened_predecessors and I couldn't think of a good way around it

Describe alternatives you've considered

Testing

Tested on my trail PR and it looks good just wanted to make this a separate PR

Actually nvm this seems to break the crossroad manholes somehow False alarm the manhole id sometimes being overwritten appears intended (or at least is happening in vanilla)

Additional context