Replace int_id<json_flag> with string_id<json_flag> (housekeeping)

[Aivean]

Summary

SUMMARY: Infrastructure "Replace int_id<json_flag> with string_id<json_flag> (housekeeping)"

Purpose of change

In #44694 I switched item flags from std::string to int_id<json_flag>, specifically to speedup has_flag lookups.
In a couple of places that resulted in ugly code, when a temporary collection of string_ids was necessary, because it was populated before int_ids are available.

After #45099 string_id is faster* than int_id for lookups in the small collections (see "string_and_int_ids_benchmark").

_{* faster compared to string_id -> int_id conversion + int_id lookup}

So currently there is no need to have the ugly code with temporary collections and all item flags can be represented as string_id<json_flag>.

Describe the solution

1. Remove all temporary collections
2. Alias flag_id to flag_str_id:

     using flag_id = string_id<json_flag>;
     using flag_str_id = flag_id;

The intension is to have a follow up PR with mass renaming of flag_str_id to flag_id.

Describe alternatives you've considered

None.

Testing

Game compiles and loads.
Smoke checked following things:

• clothing modification
• item flags saving and loading
• effect flags

Also did a profiling in the item-intensive area to ensure that no performance regressions arised.

Additional context

Profiling results (after the change), filtered by string_id<json_flag>: