Revert "Add -async-main flag to favor asynchronous main"

This reverts commit da0a3311a5.

include/swift/Basic/LangOptions.h

@@ -223,9 +223,6 @@ namespace swift {
     /// Emit a remark after loading a module.
     bool EnableModuleLoadingRemarks = false;
 
-    /// Resolve main function as though it were called from an async context
-    bool EnableAsyncMainResolution = false;
-
     ///
     /// Support for alternate usage modes
     ///