Finish removing all references to `flag_name_resolution_2_0`

[CohenArthur]

There are still some that should be removed, for exemple in the ConstChecker and UnsafeChecker implementation

[fisnikhasani]

I am assuming this is no longer an experimental feature, but more of a default implementation. Thus, the checking of the flag is no longer required.

[powerboat9]

Yep. I was working on this, before I ran into some unexpected issues on alpine CI and tabled it.

[fisnikhasani]

@powerboat9 has resolved this issue already. Close #4503.