[libadwaita/wip/exalm/fix] doc: Remove an outdated header bar split button mention

[Alexander Mikhaylenko <alexm src gnome org>]

commit 6fb3655c9780ed4b2ebb8b7b2bf7244909959b89
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Fri Sep 10 20:01:41 2021 +0500

    doc: Remove an outdated header bar split button mention
    
    Split buttons don't get background by default anymore, don't suggeste icons
    for them.

 doc/migrating-libhandy-1-4-to-libadwaita.md | 3 ---
 1 file changed, 3 deletions(-)
---
diff --git a/doc/migrating-libhandy-1-4-to-libadwaita.md b/doc/migrating-libhandy-1-4-to-libadwaita.md
index d30a77b1..d48f5d36 100644
--- a/doc/migrating-libhandy-1-4-to-libadwaita.md
+++ b/doc/migrating-libhandy-1-4-to-libadwaita.md
@@ -389,9 +389,6 @@ class and two buttons packed inside, use `AdwSplitButton` as follows:
 </object>
 ```
 
-If the split button only had text, also use `AdwButtonContent` the same way as
-for other text buttons.
-
 ### Adjusting Linked Buttons
 
 For other linked together buttons, simply stop linking them.