Remove pthread alert polyfill (#23882)

I'm not sure why we were polyfilling this web API in all our workers. We
don't generally do this for other APIs.

Author: sbc100

src/lib/libpthread.js

@@ -290,8 +290,6 @@ var LibraryPThread = {
           }
 #endif
           onFinishedLoading(worker);
-        } else if (cmd === 'alert') {
-          alert(`Thread ${d.threadId}: ${d.text}`);
         } else if (d.target === 'setimmediate') {
           // Worker wants to postMessage() to itself to implement setImmediate()
           // emulation.

src/runtime_pthread.js

@@ -64,11 +64,6 @@ if (ENVIRONMENT_IS_PTHREAD) {
   if (!Module['printErr'])
 #endif
     err = threadPrintErr;
-  function threadAlert(...args) {
-    var text = args.join(' ');
-    postMessage({cmd: 'alert', text, threadId: _pthread_self()});
-  }
-  self.alert = threadAlert;
 
   // Turn unhandled rejected promises into errors so that the main thread will be
   // notified about them.

test/other/codesize/test_codesize_minimal_pthreads.gzsize

@@ -1 +1 @@
-4108
+4054

test/other/codesize/test_codesize_minimal_pthreads.jssize

@@ -1 +1 @@
-8550
+8420