Remove some old workarounds from ConcreteDiscretePost

[schillic]

Requires:

• ~https://github.com/JuliaReach/LazySets.jl/issues/1502~
• ~https://github.com/JuliaReach/LazySets.jl/issues/1607 (waiting for new LazySets release)~
• ~https://github.com/JuliaReach/LazySets.jl/issues/1609 (waiting for new LazySets release)~

[schillic]

Tests pass locally. We just need to wait for a new LazySets release here.

[schillic]

This is now ready.