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 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013rGmDUNr1hUf4zo1X3dak8

assets/color_books/README.md

@@ -6,8 +6,7 @@ carries its source and licence in its header comment.
 | File | Book | Source and licence |
 |---|---|---|
 | `ral-classic.acb` | RAL CLASSIC, 216 approximate sRGB values | Adapted from Wikipedia, "List of RAL colours"; CC BY-SA 4.0 |
-| `sikkens-4041.acb` | SIKKENS 4041, 1,624 colours | Open Colour Systems Collection 2.0, © 2015 dtp studio oldenburg (freieFarbe e.V.); CC BY-ND 4.0. Converted from the GIMP palette: same names, values and order, only the file format changed |
 
 Values are approximate screen representations, not colour-binding
-references. RAL is a registered trademark of RAL gGmbH and Sikkens of
-AkzoNobel; these files are not affiliated with or endorsed by either.
+references. RAL is a registered trademark of RAL gGmbH; this file is not
+affiliated with or endorsed by RAL gGmbH.

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.