Tests: Re-enable async_sequence_existential.swift on Windows.

The fix in #80685 (using `-o` to write
the output to a file, instead of relying on stdout/stderr redirection) seems to
have addressed this issue on other bots.

Resolves #80582.

Author: tshortli

test/Concurrency/async_sequence_existential.swift

@@ -5,9 +5,6 @@
 
 // REQUIRES: concurrency
 
-// https://github.com/swiftlang/swift/issues/80582
-// UNSUPPORTED: OS=windows-msvc
-
 extension Error {
   func printMe() { }
 }