clean up @retroactive conformances

[weissi]

Motivation:

• #if swift is mostly wrong
• too many #ifs that were misleading and unnecessary

Modifications:

• Clean up with a single #if compiler(>=6.0)

Result:

• Nicer code

---

proof 1: Only Swift 6 has @retroactive via check for #if $RetroactiveAttribute

$ for image in swift:5.8 swift:5.9 swift:5.10 swiftlang/swift:nightly-6.0-jammy; do echo -n "does $image have @retroactive? "; echo -e '#if $RetroactiveAttribute\nprint("yes")\n#else\nprint("no")\n#endif\n' | docker run -i --rm "$image" swift -; done
does swift:5.8 have @retroactive? no
does swift:5.9 have @retroactive? no
does swift:5.10 have @retroactive? no
does swiftlang/swift:nightly-6.0-jammy have @retroactive? yes

proof 2: Only Swift 6 has @retroactive via compiling something

$ for image in swift:5.8 swift:5.9 swift:5.10 swiftlang/swift:nightly-6.0-jammy; do echo -n "does $image have @retroactive? "; echo -e 'protocol P {}\nstruct S {}\n extension S: @retroactive P {}\n' | docker run -i --rm "$image" swift - 2>/dev/null && echo yes || echo no; done
does swift:5.8 have @retroactive? no
does swift:5.9 have @retroactive? no
does swift:5.10 have @retroactive? no
does swiftlang/swift:nightly-6.0-jammy have @retroactive? yes

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See also #2718

[dnadoba]

Matches with the proposal.
IIRC, this was only necessary during the transition period.