corrected some oldIE-related comments, reverted some workarounds

I've spent some time looking for IE workarounds left in the code; I tried to test which ones might be still needed - and then update comments so that it's clear workarounds weren't needed just for oldIE - and reverted a few workarounds.

Now, I'm not 100% sure if all those decisions were correct so bare with me. I've run the full jQuery test suite and my changes didn't introduce any new bugs. I'll now add inline comments to the lines I've changed explaining my motives.

There are just a few issues I didn't know what to do about:

1) ajax.js, lines 42-52: wrapping accessing `location.href` in try-catch to account for some location properties becoming write-only after changing `document.domain`.
Bug report linked didn't have any test case and my naive attempts didn't produce anything significant. I created a simple test case: http://jsfiddle.net/ZgcNN/2/ but even IE6 passes it each time so I guess it's not that simple. Is there any way to test if this workaround is still needed? Any ideas?

2) ajax.js, lines 175-177: Exclude scripts to avoid IE 'Permission Denied' errors.
Is the mentioned error the only reason why scripts are excluded here? If so, I might try to test further but if it's not that maybe the comment should be changed.