[gtksourceview] README: remove development branch note

[Christian Hergert <chergert src gnome org>]

commit 864408d1a99b53528d87997e52aea9fa82b98900
Author: Christian Hergert <chergert redhat com>
Date:   Wed Mar 10 14:27:53 2021 -0800

    README: remove development branch note

 README.md | 2 --
 1 file changed, 2 deletions(-)
---
diff --git a/README.md b/README.md
index 2703e732..2fcd2dcf 100644
--- a/README.md
+++ b/README.md
@@ -1,8 +1,6 @@
 GtkSourceView
 =============
 
-This is a development branch progressing towards GTK 4 support.
-
 GtkSourceView is a GNOME library that extends GtkTextView, the standard GTK+
 widget for multiline text editing. GtkSourceView adds support for syntax
 highlighting, undo/redo, file loading and saving, search and replace, a