Bug 1034682 - Remove nonsensical sandboxProto waiving in the wantXrays case. r=peterv

This doesn't make sense. wantXrays means that the WrapperFactory machinery gives
us Xrays in the same-origin case. When it's false, we don't have Xrays to
same-origin objects to begin with, so the waive is useless. Its only observable
effect comes in the situation of privileged (i.e. nsEP) sandboxes with a
less-privileged sandboxPrototype. And I see no reason why these things should get
a waived prototype.

It looks like this was added in 2010 for the Web Console in bug 611401. None of
that is applicable anymore.
Bug 1034682 - Move remaining wacky wantXrays behavior to the JS entry point so that we don't have to worry about it when creating Sandboxes from C++. r=peterv
Bug 1034682 - Set wantXrays to false for content XBL scopes. r=billm