refactor: remove binary handling for the polling transport

Since Engine.IO v4, the binary payloads are always encoded as base64
with the polling transport.

See https://github.com/socketio/engine.io-protocol#difference-between-v3-and-v4

Possibly related: socketio/socket.io-client#1391

lib/transports/polling-xhr.js

@@ -308,24 +308,8 @@ class Request extends Emitter {
    * @api private
    */
   onLoad() {
-    let data;
-    try {
-      let contentType;
-      try {
-        contentType = this.xhr.getResponseHeader("Content-Type");
-      } catch (e) {}
-      if (
-        contentType === "application/octet-stream" ||
-        contentType === "application/octet-stream; charset=UTF-8"
-      ) {
-        data = this.xhr.response || this.xhr.responseText;
-      } else {
-        data = this.xhr.responseText;
-      }
-    } catch (e) {
-      this.onError(e);
-    }
-    if (null != data) {
+    const data = this.xhr.responseText;
+    if (data !== null) {
       this.onData(data);
     }
   }