Removed some IE6 specific code - saves 25 gz bytes

Some IE6 specific code was adding complexity to the `ajax.js` file. By refactoring it, for the `master` branch, we can improve readability and save 25 gz bytes.

```
Running "compare_size:files" (compare_size) task
   raw     gz Sizes
246147  72939 dist/jquery.js
 84233  29457 dist/jquery.min.js

   raw     gz Compared to master @ d837f119c3729565103005d5d7fa89e1dd8110cb
  -348   -113 dist/jquery.js
   -66    -25 dist/jquery.min.js
```
Fix for this was presented in February 2011, ticket, obviously, was created earlier (just like the comment you mentioned), IE9 was released in March of the same year.

In other words we don't know if bug persist in newer IE browsers, so i would much rather try to recreate those circumstances, even if they not stable or at worst case scenario provide some other proof, than to blindly remove stuff.