Update async_taskgroup_throw_rethrow.swift

Enable the test that was previously failing on 5.5.  This has been stable on main for a while, and seems to also be now passing on 5.5

Author: compnerd

test/Concurrency/Runtime/async_taskgroup_throw_rethrow.swift

@@ -7,8 +7,6 @@
 // REQUIRES: concurrency_runtime
 // UNSUPPORTED: back_deployment_runtime
 
-// XFAIL: OS=windows-msvc
-
 struct Boom: Error {}
 struct IgnoredBoom: Error {}