Remove the usage of `TriggerFlags.XYZSlice.signatures` in the menu
generation. The only hook to modify the list of signatures externally is
the `setChainFilter` method. So there is no need to even expose the list
of signatures outside the `TriggerMenuMT` package via flags:

- Introduce `ChainStore` class/dict that holds the list of chains per
  signature. Also takes care of only allowing a fixed set of signature
  names.
- `setupMenu()` return a `ChainStore` that is used in `GenerateMenuMT`.
- Adapt `runHLT_standalone.py` to the chain filter changes.

Closes ATR-24201.