Remove `Self: Sized` from `libsyntax::parse::lexer::Read::next_token()` function

[gyscos]

Commit 6887202 added the Self: Sized bound to the next_token function in the libsyntax::parse::lexer::Reader trait (diff here).
This bound doesn't seem necessary (it compiles fine without it), but it prevents using this method from a trait object.

Is there any reason for its addition? If not, could we remove it?

[mitaa]

That must be a leftover from me trying to make unwrap_or_abort take a closure somehow work.. sorry.

#33199 (comment)

[Mark-Simulacrum]

I'm marking this as E-easy because all that needs to be done is to remove Self: Sized from here, I believe.

[tommyip]

I would like to take this issue.

[Mark-Simulacrum]

Go ahead! Let us know what we can do to help you. You can reach out either here or on IRC (#rustc on irc.mozilla.org).