Revisit how we remove unused things from the ast

[rvcas]

At the moment the implementation for removing unused things from a typed expr sequence is not ideal when you consider how the LSP identifies where things are located. In general we MUST make sure any type-checking-time AST transforms we do don't mess with the LSP being able to "see" that code.

let _ = some_function()

in the example above you cannot go to definitions properly for some_function