[gtksourceview] build: remove old meson_postinstall.py

[Christian Hergert <chergert src gnome org>]

commit 9109702efea1477128d1d84a1844f6c0439c84b0
Author: Christian Hergert <chergert redhat com>
Date:   Wed Feb 2 19:31:32 2022 -0800

    build: remove old meson_postinstall.py
    
    We require a newer version of meson now, so we can drop this.

 meson.build          |  8 +-------
 meson_postinstall.py | 13 -------------
 2 files changed, 1 insertion(+), 20 deletions(-)
---
diff --git a/meson.build b/meson.build
index 5335cee7..60598980 100644
--- a/meson.build
+++ b/meson.build
@@ -343,10 +343,4 @@ summary({
   }, bool_yn: true,
 )
 
-if meson.version().version_compare('>=0.57.0')
-  gnome.post_install(
-    gtk_update_icon_cache: true,
-  )
-else
-  meson.add_install_script('meson_postinstall.py')
-endif
+gnome.post_install(gtk_update_icon_cache: true)