Remove unnecessary await in MODULARIZE=instance mode. NFC (#23219)

Since the init function is already marks as async it fine/better to
simply return the promise here rather than awaiting on it.

tools/link.py

@@ -2403,7 +2403,7 @@ def modularize():
 
 %(generated_js)s
 
-  return await moduleRtn;
+  return moduleRtn;
 }
 ''' % {
       'generated_js': generated_js