Remove unneeded code

`gemini-1.5-pro-latest` successfully accept multiple user messages,
so it's now no need to insert dummy model messages between them

src/worker.mjs

@@ -197,19 +197,14 @@ const transformMsg = async ({ role, content }) => {
 const transformMessages = async (messages) => {
   const contents = [];
   let system_instruction;
-  let lastRole;
   for (const item of messages) {
     if (item.role === "system") {
       delete item.role;
       system_instruction = await transformMsg(item);
-      continue;
-    }
-    item.role = item.role === "assistant" ? "model" : "user";
-    if (item.role === "user" && lastRole === "user") {
-      contents.push({ role: "model", parts: { text: "" } });
+    } else {
+      item.role = item.role === "assistant" ? "model" : "user";
+      contents.push(await transformMsg(item));
     }
-    lastRole = item.role;
-    contents.push(await transformMsg(item));
   }
   if (system_instruction && contents.length === 0) {
     contents.push({ role: "user", parts: { text: "" } });