remove unnecessary framePtr code (#19645)

lib/system/jssys.nim

@@ -130,7 +130,8 @@ proc unhandledException(e: ref Exception) {.
   when NimStackTrace:
     add(buf, rawWriteStackTrace())
   let cbuf = cstring(buf)
-  framePtr = nil
+  when NimStackTrace:
+    framePtr = nil
   {.emit: """
   if (typeof(Error) !== "undefined") {
     throw new Error(`cbuf`);