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

This reverts commit da0a3311a5.

lib/Frontend/CompilerInvocation.cpp

@@ -497,8 +497,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Diags.diagnose(SourceLoc(), diag::warn_flag_deprecated,
                    "-enable-experimental-async-top-level");
 
-  Opts.EnableAsyncMainResolution = Args.hasArg(OPT_async_main);
-
   Opts.DiagnoseInvalidEphemeralnessAsError |=
       Args.hasArg(OPT_enable_invalid_ephemeralness_as_error);