Select Color: ship only RAL CLASSIC as a built-in colour book

SIKKENS 4041 is no longer built in; it can still be loaded from an .acb
file in a colour book folder.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_013rGmDUNr1hUf4zo1X3dak8

src/ui/color_select/book.rs

@@ -57,15 +57,10 @@ impl ColorBook {
     }
 }
 
-/// The colour books that ship with the program; their licences are in the
-/// files' header comments.
+/// The colour book that ships with the program; its licence is in the
+/// file's header comment.
 pub fn builtin() -> impl Iterator<Item = ColorBook> {
-    [
-        include_str!("../../../assets/color_books/ral-classic.acb"),
-        include_str!("../../../assets/color_books/sikkens-4041.acb"),
-    ]
-    .into_iter()
-    .filter_map(|text| parse(text).ok())
+    parse(include_str!("../../../assets/color_books/ral-classic.acb")).into_iter()
 }
 
 /// Read an `.acb` file's text.