`make_tokenstream` has three commented hacks, and a comment at the top
referring to rust-lang#67062. These hacks have no observable effect, at least as judged
by running the test suite. The hacks were added in rust-lang#82608, with an explanation
[here](rust-lang#82608 (comment)). It
appears that one of the following is true: (a) they never did anything useful,
(b) they do something useful but we have no test coverage for them, or (c)
something has changed in the meantime that means they are no longer necessary.

This commit removes the hacks and the comments, in the hope that (b) is not
true.

These seem to have no useful effect... they don't seem useful from a
code inspection point of view, and they affect anything in the test
suite.