Revert "go/ast: clarify when Ident.Obj is nil"

This reverts commit 52aef05498.

Reason for revert: After discussion on CL 347530, it is not clear this is an improvement to the documentation.

Updates #48141

Change-Id: I5f3d9995c5f5666b92602c4b8ec393673baa73fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/347592
Trust: Cherry Mui <cherryyz@google.com>
Trust: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

src/go/ast/ast.go

@@ -290,7 +290,7 @@ type (
 	Ident struct {
 		NamePos token.Pos // identifier position
 		Name    string    // identifier name
-		Obj     *Object   // denoted object; or nil if the referenced declaration is not found in the parsed scope
+		Obj     *Object   // denoted object; or nil
 	}
 
 	// An Ellipsis node stands for the "..." type in a