[stdlib] remove preconditions from compatibility entry point
- The old implementation of `UnsafePointer.withMemoryRebound` had no preconditions.
- When implementing SE-0333, we forwarded the old entry point to the new implementation, which has preconditions.
- This change removes the precondition from the compatibility entry point, reverting it to its previous behaviour.
- This resolves rdar://90462471
