Remove unique ETag hack as it is no longer necessary

Whilst porting some of our HTTP header handling to the kernel patch, I ran into this little gem:

http://api.drupal.org/api/drupal/includes!bootstrap.inc/function/drupal_page_header/7

Have a look at the huge paragraph there. Now read it again to try and understand what it's doing. It may take several tries. :-)

Now, have a look at the Firefox bug that it links to: https://bugzilla.mozilla.org/show_bug.cgi?id=269303

That was marked as a duplicate of another issue in 2009: https://bugzilla.mozilla.org/show_bug.cgi?id=510359

That issue was also resolved in 2009. So it will have been fixed for ~4 years by the time Drupal ships.

OK, but does that mean we don't need to support that browser version?

According to Firefox's release history, the bug would have been fixed before Firefox 3.6 shipped, and was likely also backported to Firefox 3.5. That means the only extant versions of Firefox with this bug should be 3.0 and earlier. Firefox 3.0's market share is, I think, in negative numbers at this point as Mozilla just announced that they have enabled auto-update for Firefox 3.6, so all Firefox 3.6 browsers will overnight turn into Firefox 13 in the not too distant future.

That is, browsers affected by that bug no longer exist in the wild.

We should therefore drop that hack entirely, and free up the use of ETags for something useful, like better cache control.

However, let us NOT do that until after the #1463656: Add a Drupal kernel; leverage HttpFoundation and HttpKernel lands, because I don't want yet another merge conflict. :-) It should be easy to strip out afterward.