refactor(library/logic): cleanup some of the proofs in cast.lean, remove piext axiom

Remark: the main motivation for piext was Lean 0.1 simplifier.
We are using a different approach in Lean 0.2.
The axiom is not needed anymore.
It is also not used in any part of the standard library
refactor(library/logic): cleanup some of the proofs in cast.lean, remove piext axiom