remove redundant pre-concurrency flag (#314)

motivation: address warnings

changes:
* remove pre-concurrency flag when not longer needed

Author: tomerd

Sources/AWSLambdaRuntimeCore/LambdaContext.swift

@@ -12,7 +12,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#if swift(<5.9)
 @preconcurrency import Dispatch
+#else
+import Dispatch
+#endif
+
 import Logging
 import NIOCore