[Lean] Disable spec-trait phase for Lean and remove pre/postcondition checks

[abentkamp]

We can inline pre/post conditions in the contracts of traits. Then we can disable the spec-trait phase in Lean and these checks are probably no longer needed:

hax/rust-engine/src/backends/lean.rs

Line 1216 in ce58911

item.ident.is_precondition() || item.ident.is_postcondition() ||

hax/rust-engine/src/backends/lean.rs

Line 1296 in ce58911

item.ident.is_precondition() || item.ident.is_postcondition() ||