Always use the older available for concurrency in tests

Author: DougGregor

test/lit.cfg

@@ -407,11 +407,7 @@ lit_config.note('Compiling with -swift-version ' + swift_version)
 config.swift_test_options = '-swift-version ' + swift_version
 
 # Define a macro for the next release OS version.
-# This macro can be used in code as `@available(SwiftStdlib 5.5, *)`.
-if 'back_deploy_concurrency' in config.available_features:
-    swift_stdlib_macro = '\'SwiftStdlib 5.5:macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0\''
-else:
-    swift_stdlib_macro = '\'SwiftStdlib 5.5:macOS 12.0, iOS 15.0, watchOS 8.0, tvOS 15.0\''
+swift_stdlib_macro = '\'SwiftStdlib 5.5:macOS 10.15, iOS 13.0, watchOS 6.0, tvOS 13.0\''
 config.swift_frontend_test_options += ' -define-availability ' + swift_stdlib_macro
 config.swift_driver_test_options += ' -Xfrontend -define-availability -Xfrontend ' + swift_stdlib_macro