Removed oldIE support from Sizzle - saves 238 gz bytes

By removing oldIE support from 2.1+'s Sizzle, the code becomes more readable, and the filesize is reduced.

```
Running "compare_size:files" (compare_size) task
   raw     gz Sizes
244350  72434 dist/jquery.js
 83434  29244 dist/jquery.min.js

   raw     gz Compared to master @ d837f119c3729565103005d5d7fa89e1dd8110cb
 -2145   -618 dist/jquery.js
  -865   -238 dist/jquery.min.js
```

Cheers,
Chris