Remove an erroneous comment.

The comment just below this one describes how the `impl !Send for
FatalError` makes it impossible to `panic!(FatalError)`.

(Which still puzzles me, because `panic!(FatalError)` is invalid because
the first argument of `panic!` must be a string literal. And
`panic!("{}", FatalError)` works whether `Send` is impl'd or not.)

Author: nnethercote

compiler/rustc_span/src/fatal_error.rs

@@ -1,6 +1,4 @@
-/// Used as a return value to signify a fatal error occurred. (It is also
-/// used as the argument to panic at the moment, but that will eventually
-/// not be true.)
+/// Used as a return value to signify a fatal error occurred.
 #[derive(Copy, Clone, Debug)]
 #[must_use]
 pub struct FatalError;