cue/ast: remove outdated docs

There is currently, and probably never has been,
a `Comments` list in the top level `ast.File` struct.
This is probably a remnant of the original Go implementation,
so remove it.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I64763b9b206f619cd083503bd16fe68c92c13342
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1205764
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cue/ast/ast.go

@@ -945,10 +945,6 @@ func (d *EmbedDecl) End() token.Pos { return d.Expr.End() }
 // Files and packages
 
 // A File node represents a CUE source file.
-//
-// The Comments list contains all comments in the source file in order of
-// appearance, including the comments that are pointed to from other nodes
-// via Doc and Comment fields.
 type File struct {
 	Filename string
 	Decls    []Decl // top-level declarations; or nil