Cleanup: modify `RemoveLet` to have two separate spans, one for the `let` and another for the `let ` removal suggestion so that the primary span doesn't point at unnecessary whitespace

[jieyouxu]

Ideally we'd modify RemoveLet to have two separate spans, one for the let and another for the let removal suggestion so that the primary span doesn't point at unnecessary whitespace. We can address that as follow up.

Originally posted by @estebank in #132996 (comment)

[tyrone-wu]

@rustbot claim