Concurrency: Revert workarounds for `@backDeployed` condfails.
All compilers that must be able to compile the standard library's textual
interface accept the following:
- back deployed functions declared without `@available`
- `@inlinable` back deployed functions
- `defer` blocks in back deployed functions
We can therefore revert the workarounds added in
https://github.com/apple/swift/pull/62946/ and
https://github.com/apple/swift/pull/62928/.
Resolves rdar://104085810
