Remove unused PThread.mainRuntimeThread. NFC (#17016)

The last usage was removed in #17017.

Author: sbc100

src/library_pthread.js

@@ -599,9 +599,6 @@ var LibraryPThread = {
       /*start_profiling=*/true
 #endif
     );
-#if ASSERTIONS
-    PThread.mainRuntimeThread = true;
-#endif
     PThread.threadInit();
   },