cleanup pre Swift 5.6 code

[sebsto]

The code base uses pre compiler directives to selectively add Sendable protocol to struct when compiling on Swift >= 5.6

#if swift(>=5.6)
extension XXX: Sendable
#endif

This PR removes this old code and moves all Sendable definitions to the same line as the struct definition.

public struct XXX: Sendable

[adam-fowler]

We're good