Don't mark proxy async funcs as Asyncify

In #19093 both async functions and functions marked with `NAME__async: true` were marked as functions to be processed by Asyncify.

However, looking at usages, standalone async functions are currently used with emscripten_proxy_finish-based callbacks and not with Asyncify.

Besides, such functions can't work with Asyncify unless we add automatic `Asyncify.handleAsync` wrapping around each such function as well. We could do that, but given that there are already is some confusion around what async means for any particular function, it seems better to keep explicit calls to `emscripten_proxy_finish` and `Asyncify.handleAsync` correspondingly.

Hence, in this PR I'm not adding more automatic wrapping but instead just removing the assumption for standalone async functions which should somewhat reduce code size as they will no longer be Asyncify-ed when not necessary.

Author: RReverser

src/jsifier.js

@@ -326,14 +326,9 @@ function(${args}) {
         deps.push('setTempRet0');
       }
 
-      let isAsyncFunction = false;
       if (ASYNCIFY) {
         const original = LibraryManager.library[symbol];
-        if (typeof original == 'function' ) {
-          isAsyncFunction = LibraryManager.library[symbol + '__async'] ||
-                            original.constructor.name == 'AsyncFunction'
-        }
-        if (isAsyncFunction) {
+        if (typeof original == 'function' && LibraryManager.library[symbol + '__async']) {
           asyncFuncs.push(symbol);
         }
       }