doc_formatter: Remove a resolved TODO

This TODO was resolved when we switched to tree-house. Tree-house's
`Highlight` type is a `Option<NonZeroU32>`, so `GraphemeSource` is now
8 bytes in size, as the TODO requests (for a 64-bit arch).

Author: the-mikedavis

helix-core/src/doc_formatter.rs

@@ -26,7 +26,6 @@ use crate::syntax::Highlight;
 use crate::text_annotations::TextAnnotations;
 use crate::{Position, RopeSlice};
 
-/// TODO make Highlight a u32 to reduce the size of this enum to a single word.
 #[derive(Debug, Clone, Copy)]
 pub enum GraphemeSource {
     Document {